To help you decide rim size and motor, play around with the hubmotor simulator at different voltages:
http://www.ebikes.ca/simulator/
Real-world peak speed occurs right around the green, efficiently-line peak.
If you're planning higher voltages, like 72 volts, and aren't planning to ride over 40mph, then it makes more sense from a power and efficiency perspective to choose a higher-numbered X5 in a smaller rim. At 72 volts, my 5304 in a 24" rim had a top speed of 44mph. In a 20" rim, it has a top speed of 35mph.