For "standard" bulkheads.......
1/2" bulkhead requires a 1-1/8 diameter hole.
3/4" bulkhead requires a 1-7/16" - 1-1/2" diameter hole.
1" bulkhead requires a 1-3/4" diameter hole.

But I've had bulkheads that were not the "norm". So, always get your bulkheads in your possession before you drill.