I would look at this forum page for information about drive train ratios and wheels.
Generally for this game ive seen robots have around mid-60s and above inches/second drive trains. You can convert to inches per second by calculating your rps (rotations per second for your wheel, then multiplying that by your wheel circumfrence).
However be wary that the faster a drivetrain is the more skillful your drive will have to be to deal with the high accelerations.
If you want a recommendation, try 450 on 3.25". Our team ran that for our early season robot and it ran quite well.