This is an ambiguous question since it doesn't give the context as to the type of size being requested. Do you mean physical? A desktop PC is around 9" wide, 18" deep, and about 15" tall. If you mean the storage capacity, it can range from almost none to several terabytes or more. If you mean the RAM capacity, it ranges from 1k of a few old computers to 64 Gb or more.
So it is hard to be more specific with a question this broad.