Summary

There are 3 sizes of wall plates:

Standard (2.75 by 4.5 inches)
Preferred (3.13 by 4.88 inches)
Oversized or Jumbo (3.5 by 5.25 inches)

Standard is the smallest size and is normally used in newer homes where tolerances are tighter. Preferred is a little bit bigger than Standard and can be used to cover up gaps. Oversized can be used for large gaps.