There is not a typical size for electric motors, as the uses for these motors vary so widely. The National Electric Manufacturers Association lists 164 official frame designations, most of which are different in at least one dimension from the others.
Diameters of standard motors range from 2 5/8 inches to 9 inches, according to NEMA tables from 1984. The various other measurements, including mounting and fitting dimensions, are also listed, and all of these must be matched exactly if one motor is to be replaced with another from a different manufacturer. Each of the measurements has a designated letter in the official tables to match motors more efficiently.