This gets tricky.
There are two types of bulkheads generally used in the aquarium hobby.
The first is the standard Schedule 80 bulkhead fitting.
The second is a lightweight bulkhead fitting.
See them both here -
Savko Plastic Pipe & Fittings
That page also lists the size holes you need for both types of bulkheads.
The lightweight bulkhead fitting has a thinner wall, and does not require as large a hols as the Schedule 80 bulkheads do. However the schedule 80 bulkhead is a lot stronger, and more expensive.