I don't know the actual screw size, but by measuring an actual case screw (from an Antec case fan package), it looks like a 4mm hole is teh win.

Do you need to make an extra fan mount in your PC case? Then you've come to the right place. In this guide, we'll show you how to create your own fan mount, by cutting a fan blowhole and then ...

The screws should make their own threads. Just think of a tap and die set. The tap makes the threads into a hole, the die makes the threads onto a screw or bolt.....I ...