The width of the wheel will be dictated by the width of the hull and the space needed on either side for the cranks and pitman arms that drive the wheel, so perhaps 140mm wide?
The diameter will depend on where the wheel is mounted in relation to the models waterline... But at a guess maybe 90-100mm
Revs don't need to be as high as a lot of people imagine, maybe 80-100RPM at full speed.
Regards
Eddy