The first thing you need to do is figure out what knee angle feels good to you. In general for recumbent bikes a maximum knee angle of around 90 degree is best ergonomically. Less than 80 degrees may hurt your knees. More than 90 degrees may be less than optimum from a power standpoint. Exactly what angle feels good for you is a function of how strong your knees joints are, as angles below 90 degrees can stress your knees more.

Instructions:
To find out what is optimal for you, just whip out your tape rule and make some measurements (in centimeters). For the "A" measurement, just measure from the side of your hip where you think your hip-joint is, to the side of your knee where it flexes.  The "B" measurement is from the side of your knee where it flexes to the ball of your foot (dashed red line in drawing below).  Enter your measurements and vary the crank length until your knee angle is around 85 degrees.
 

Hip to knee and knee to pedal measurements

Note that in general people feel more comfortable with shorter cranks on higher BB bikes, and longer cranks on lower BB bikes, so for a Tour Easy maybe go for 82 degrees and on a low or high racer maybe go for 89 degrees.

I'm tall, and my hip to knee + knee to pedal = 112cm. Through experimentation, 165mm are the shortest cranks that feel good for me, and this is reflected in this calculator, as 165mm gets me to 89 degrees. A shorter person with an 80 cm total hip to pedal length would need to go to 145mm cranks just to keep the knee angle greater than 80 degrees.

For traditional upright bikes a knee angle of greater than 80 degree is recommended, which explains why all upright bikes come with 175mm cranks.

An article by James Martin PHD notes that human power output remains relatively consistent usingh a wide range of rank lengths.

Though good results have been reported for crank lengths down to 110mm, crank RPMs need to be higher for shorter cranks to produce the same power as longer cranks, so there is a diminishing return at the point where you can't crank any faster. Your mileage may vary...

This program uses the law of cosines to calculate the knee angle given three sides of a triangle: