Anybody know what size drill bit to use to install a J hook on a trap?

Depends on the size of the j-hook. They come in different sizes.

You will need to drill it oversize for clearance so it will swivel. Just hold a bit against the hook, and look for one the same size-ish, and go a 1/32" over that.
If you have a bit index, try slipping the hook end into the holes the drills fit in, till it won't go, then add the 1/32".