Dog head has a point. The real answer depends on how you have your shoes adjusted. With correct "major" or "minor" shoe adjustments, the pistons should move very little each time the pedal is pushed.
If you have only done a "minor" adjustment in the past or never had your drums off then you could have a great deal of movement and a very low pedal. Not trying to be rude or anything, but a study of the -20 manual really is needed to understand the "major" and "minor" maintenance routine. They even have pictures.
Now, to try and answer your question. Each wheel cylinder has a bore of 1-3/8". To determine volume, you need the length as well. I don't have a wheel cylinder apart right now, so I can't measure that for you. However, I can't think of a reason except for figuring out how much fluid to put in the system from a dry start why you would need that figure. Just for fun, figure a quart of brake fluid to go from all dry to full and bled.
