Which head size is right for my banjo?

1. Measure the banjo rim 

This should be done across the top of the rim, after removing the old head and tension hoop. Make sure to take at least two measurements of the rim's outside diameter across the rim at different points.  Get an average of the measurements to arrive at the proper head size you'll need.  

2. Choosing the proper diameter size 

Banjo heads are available in16th of an inch diameter increments.  So if your rim measurement falls between any of those increments, it's better to choose the larger head size of the two.  A slightly larger (1/32" or even 1/16" larger) head would fit while a head 1/16" too small will not.

3. Choosing the proper crown height

Crown height (also called collar height) is the distance from the top of the head's mounting band to the "crown" (top surface) of the head.  Most banjos will do well with either a medium or high crown head.  Banjos with arched tops, whether with an archtop tone ring or an integral arched top, will require a low crown head.  Also, banjos that utilize a short tension hoop (3/8" tall, as most low end Asian-made banjos do, for example) do better with a low crown head.
Too low a crown will result in the tension hoop not pulling down far enough as to provide enough clearance for the strings.  Too high a crown will cause the tension hoop to be pulled down too far.