There's a table to determine flow thru bulkheads on p.93 of The Reef Aquarium, Vol. 3. Do you need permission to post all the information here?
Example. A 24" high, 1" diameter pipe will pass approx. 22.8 gpm.
The hole size is determined by the bulkhead size with some minor variations. It may vary depending on manufacturer and the schedule. (I've seen different hole sizes specified for same size fitting).
Here's one link:
Savko Plastic Pipe & Fittings
Here's another:
PVC bulkhead fittings - PlumbingStore.com