I used a 1/8", but a 3/16" would have also worked. As Red01 said, something big enough to thread a small screw in.
On mine, two came out with the drill bilt only. Two needed a screw threaded in.
Oh...before I forget, wrap a few turns of tape over the drill bit, about 3/16" from the end to use as a stop. You don't want the drill too deep!!! This IS important...