2.5" drives: iBook, PowerBook, MacBook Pro, Mac mini
3.5" drives: iMac, eMac, PowerMac
Basically, the 2.5" drives are "notebook-size", but they're also used in the Mac mini, because they wanted to create a really small package. The disks are _also_ used in some external (Firewire, USB) drive enclosures that intend to be really small. Can you tell what you need to know this for?
Btw.: There's also 1.8" drives. Those are used in harddrive-based iPods, for example.