Well, I don't see the one I used and it's been so long I can't remember the exact size. I know I bought a pack of them, and used the 3 1/2 or 4 inch one.
http://www.homedepot.com/Tools-Hardware ... ogId=10053
You need a good drill with high torque, too. MAKE SURE you remove every single thing from your case, as cutting through the metal will fling fillings all over. I taped cloth over the 250mm top fan to prevent debris getting in there and later falling back down to short-circuit my motherboard.
And by god, clean every single filling out after you're done. Used compressed air.
EDIT: I used a bit sander to smooth out the edges of my holes. They're like nine dollars. Don't want to cut your cables.
Drill on one side of the case until you feel like you're 4/5ths of the way through, then flip it over and finish the cutout from the other side. It's a cleaner hole that way.