How big is a framed opening for a door relative to the finished door opening width?
Normally rough opening is 2 inches larger then the nominal door size. This leaves approximately 1/4" on each side for shimming to plumb. 

Some carpenters prefer 2-1/2", leaving a 1/2" gap for shimming. In case framing isn't very precise (say when using unskilled volunteer labor), 1/4" can sometimes be inadequate.