3.5" drives generally use size 6-32 screws, 0.15" to 0.25" in length. This is also the size used to hold expansion cards in the slots in an ATX case.
2.5" drives, optical and floppy drives, generally use metric M3 screws, about a half a centimeter in length.
Go down to your local hardware store (these are very common screw thread sizes, although short lengths can sometimes be hard to find) and buy yourself a few dozen of each, #2 Philips head (and flat, not tapered), and you'll be prepared for pretty much every situation involving storage devices and peripheral hardware.
The kit that smorizio posted is an easy, quick buy that has basically everything you'd ever need for component installation work in it. A hardware store will be...