It depends on what bulkheads you have, the normal lightweight aquarium bulkheads take a 1 3/4 inch hole, the heavy duty bulkheads can take a 2 inch or 2 1/2 inch depending on the manufacture. The best thing you can do is get the bulkhead first and then drill the hole to fit the bulkhead, rather than drill the hole first and hope the bulkhead fits the hole.
Kim