Like all plumbing parts, the listed sizes for bulkheads relate to the inside diameter of the pipe that can thread into it.
http://www.marinedepot.com/aquarium_..._strainers.asp
if you use that chart, your closest size would be a 1 1/2" bulkhead. Hopefully your holes are 2 5/8" rather than 2 1/2".
http://www.reefcentral.com/calc/drain.php
if you use that calculator, it will tell you that 1300 gph is the maximum you'd want to flow through a 1 1/2" drain.