SpokePOV can fit either BMX (20") wheels or mountain/roadoad bike (700c/37") wheels. The short version is 6.5" (16.5cm) long, the longer version is 9" (23cm).
If you are using BMX wheels, simply cut the PCB before soldering in parts and then don't solder in the extra LEDs or latches.
On a mountainbike wheel, I have found that with 1 SpokePOV you should bike 15mph/24kph. With 2 SpokePOV, bike 10mph/16kph. With 3 SpokePOVs, 7mph/11kph. This will give a nice persistant image. The faster the better, of course.
Larger wheels you might have to bike a little faster. Smaller wheels, you can go slower.
SpokePOV has been tested to work at 30mph (but can probably go even faster).