When a customer is considering a power meter or crank purchase one of the most common questions I am asked is which crank arm length or gear choice is "best."  This is usually followed by an "I heard that..." or "my coach said.." crank length/gearing is the "best".  The truth is that these choices and decisions are as varied as cyclists.  There are some basics that will get you in the right ball park, but making a choice between a 170mm or 172.5mm crank arm, or 11-23 vs 12-25 cassette requires data and experimentation, by the individual cyclist.  Generalizations have no place here as there are a large number and wide variety of factors that will determine what gearing choice and crank arm length is best.  But ultimately the goal should be determining the right combination of these choices that produces the highest amount of power (wattage) at the lowest metabolic cost (heart rate).  The good news is that this type of experimentation does not require a clinical setting.  With the advent of power meters, heart rate monitors, and a myriad of uploadable data points a cyclist can experiment with some of these factors and gather enough data over time to make educated decisions.  This will require a level of patience and diligence.  If you thought this article was going to roll this up for you I am afraid you will be disappointed.  But I will refrain from going into a complex scientific dissertation and try to keep things on an actionable level, and keep this article specific to road cyclists.

What are some of these "factors' that will determine the highest power to lowest metabolic cost?  To name just a few...

• Biological and Biomechanical Factors: Femur length, foot length, insertion point of tendons, muscle fiber make up (slow vs fast twitch), Q factor, and body asymmetry will all affect your mechanical advantage (crank arm length) and gearing.  Even if you assume two riders with the exact same body proportions (height, bone length, foot size, etc.) but one being endomorphic (large muscled) and the other ectomorphic (slight stature) their optimal crank length and gear choices may be completely different.  This is why making choices based solely on height or inseam length may not yield the most efficient results.  
• Fit and Position Factors: Changing gearing or crank length on a poorly fitted bike is putting the cart way before the horse. Crank length is the length of your lever arm, and generally speaking a longer lever arm produces more mechanical advantage.  But there is obviously a point of diminishing returns or every cyclist would be riding long crank arms.  A crank arm that is too long produces more dead spots in the pedal stroke and an choppy pedal stroke.  There may be more mechanical advantage but under high load power can decrease.  Your knees may also be poking you in the rib cage.  But some of the same disadvantages occur with a saddle that is too low.  A rider that is positioned too high, low, behind or in front of the bottom bracket is not efficient.  A rider that is assuming a position too aggressive for their flexibility will not be efficient.  A rider with ill positioned cleats will not be efficient.  A rider that is uncomfortable will not be efficient.  You get the point; if you are not fitted correctly changing mechanical factors will not improve your efficiency.   
• External factors:  The type of competition or riding you do has a tremendous affect on your mechanical choices.  High cadence criterium racing, road touring, triathlon, and time trialing all have different demands.  Crits involve rapid acceleration/deceleration of the crank, touring requires metering your energy out most efficiently over long distances, triathletes have to run off the bike, and in a time trial you want to use up every ounce of energy over the distance.  You may be training for a mountain climb or flat Ironman requiring different gear ratios.  The cadence you ride is a huge factor in gearing selection.  A rider the assumes a natural cadence of 75rpm vs a rider that averages 90rpm will likely need a different gear ratio.  

Now that you are overwhelmed by the number of factors affecting crank arm length and gearing lets begin to narrow it down.  Let us assume you have been professionally fitted and are comfortable on the bike.  Let us also assume your bike is the right size.  The crank size that came on your bike was predicated by the size of the bike.  The smaller the frame size the shorter the crank arm length.  This operates off the assumption of inseam length being a strong determining factor of crank length; which is not an altogether bad place to start.  A small rider on a 50cm bike (usually small in t-shirt sizing) would likely be inefficient with a 175mm crank length, which is why most size small bikes come with a 170mm.  However, femur length is also a strong determining factor of crank arm length.  The femur is a lever arm and a longer femur may be able to push a longer crank arm efficiently.  The length of the femur can be hard to measure accurately but if you are a long legged rider or have a short torso/long leg ratio you may want to try going up a crank arm length.

For triathletes, especially ultra distance triathletes, there is a tendency to size down in crank arm length.  The reasoning for this being a shorter crank arm keeps the hip angle more open allowing a more aggressive aero position.  A short crank arm/higher cadence also shifts more load to the aerobic system and puts less stress on the muscular system allowing fresher legs for that run off the bike.  This again will require experimentation and adjustment but I do not recommend going down more than a size and collecting data.  Going from a 172.5 to 162.5mm crank arm length may have you spinning vs mashing, but your wattage may be thrown out the window in the process.  And if you are a large muscled, endomorphic cyclists you may be working opposite your advantage.  A shorter crank arm will require a slightly faster cadence in the same gear and you may find yourself pedaling faster to achieve the same wattage.

Your most efficient gearing is a bit easier to play with and determine.  For starters there are just three common big ring combinations for road cyclists: 53/39, 52/36, and 50/34 as well as few 1x options.  Think of your big ring or rings as the starting point and your cassette as the fine tuning.  Like crank arm length, manufacturers tend to choose gearing based on frame size.  This again operates off the assumption that a smaller rider will produce less wattage, thus need a lower gear ratio.  A higher gear ratio means more speed, but only if you can turn the crank!  What you are gunning for is the gear ratio that allows you to utilize the most gears.  In other words if there are a bunch of gear combinations you never utilize because they are too high or too low your 22 speed bike may actually be 18 or 16.  For most riders, even big riders, a 53/39 is too high.  Being big does not mean you are able to produce a lot of power and the average cyclist is not able to push a high gear ratio efficiently.  I commonly hear a cyclists being concerned they will "spin out" a lower combination or run out of gears.  Pedaling between 80 and 90 rpm as most cyclists do, you would need to be going about 30-35 mph to spin out a 53/11, a high combination.  That is not a sustainable speed obviously and this ratio would be reserved for the steeper descents.  But in most cases you are more efficient making yourself as small as possible and coasting vs attempting to pedal through a fast descent.  You end up using a lot of energy for little gain in speed.  I have recommended a 50/34 or compact crank to many larger riders over the years, and I have never had one go back to more teeth in the front.  As a very rough estimate if you are not able to average over 180 watts on your endurance rides you should not be riding a 53/39.   

Your cassette is the cheapest/easiest place to experiment with gear ratios, but I recommend choosing your crank first.  A cassette change can cost you well under $100 and unless you go to a cassette with more than 28 teeth, in which case you may need a new rear derailleur.  Ask yourself if you run out of gears quickly when climbing and wish you had more, or if on the flats you are over your natural cadence looking for another gear.  This will help you determine if you want to go a higher or lower number of teeth.  A typical change may be going from an 11-23 to a 12-25 with the goal being better utilization or more utilization of gears.  You can also change your front ring(s), however this will require shortening/lengthening your chain, adjusting front derailleur height, and may require a complete change of your crank.  

Choosing your crank and your cassette is step 1, experimenting with different crank lengths is a more difficult and costly process as it requires a complete change of crank.  How to test crank arm length on your own?  There are fit tools and cranks that allow you to adjust length but they are expensive and mainly designed for trainer use.  Your fitter may have a fit bike that allows you to experiment with different crank lengths and test wattage, but this can be hard to determine in a single session.   On your own will need a heart rate monitor, a power meter, and several weeks worth of comparable data.  When comparing rides you would want to keep the format as close as possible; course, conditions, etc., or use an indoor trainer.  What you will be looking for is a lower metabolic cost (heart rate) at the same or greater wattage.  Again, this will be hard to determine with a single ride which is why you will need comparable data over time.  Don't discount feel, if the shorter or longer crank feels more efficient it probably is.  If you have the ability to go back to your old length and compare even better.  Although this article referenced road cycling the same principle can be applied to any type of cycling.

Cyclist often have a herd mentality.  Over the years I have witnessed a trend towards longer crank arms and shorter crank arms, compact cranks and over sized cranks based mainly on what a particular pro was running when they won the race.  Determine and analyze what changes you need to make to your own drivetrain, not what somebody else is running or what someone tells you is better.