30G OverFlow.

[bignose]

A 1" drain with 3/4" returns should be fine. Personally I would only have a single overflow box especially in a 30G tank. Try and keep as much room for everything else.

[HaZRaTTy]

It would be a single box with 2 - 3/4 drains to my sump - and a 1inch return back into the tank on the other side.