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.

So I'm building my first PC ever. My mobo didn't come with screws and my case didn't have an accessory case -- no standoffs, manual, nothin'. I'm in a bind since I'm new to DIY, and I don't even know ...