I have two 36" holes drilled for my calfo. That should give me 2 X 2" drains after the bulkheads go in. This is for a 5 foot long calfo. Location depends on where you want the water level to be. A calfo usually doesn't have teeth on it like a regular overflow to provide a thinner layer of water skimming. Snails or other "chunky" things shouldn't be a problem for drains as larger as you are talking about, but you might want to consider what you are going to use as a screen on the calfo. Pete or Dan can probably help you out there. They both built calfos for their tanks and build them for others.