[Espeefan]

That's some nice machine work!

For Banebots planetaries, the 540/550 sized gearboxes are 1.5 square, O.D. Length varies with the number of stages. I have a 256:1 sitting right here and it's 2 and 13/16" long. That's just the gearbox. Motors add length. You might have to go with something smaller. They make some 390 sized planetaries, but I don't know the dimensions. They are really good about listing all the major dimensions you need, right online.

I'd suggest checking out Servo City's precision gearhead motors too. They are more compact, run on 12-24 volts and can be purchased in a variety of RPMs.

You can always mix and match Banebots planetaries with different motors to get a good workable RPM range. Just need to do some simple math and figure out what the motor's Kv rating is. Kv = RPM/volt.