my reef octopus varios6 comes with a 1 1/4" return and i would reduce down to 1". Bigger pipes has less resistance so you would get more flow out of the pipes and if you have a dc pump which isn't pressure rated vs an ac pump which works better under pressure and/or restricted. So i guess what I am trying to say is that it depends on your pump type ac vs dc and or if the pump is over sized for the tank or not. Also if you have a lot of manifolds and bends. With that said keep it simple.
If I had four 1 1/2" bulkhead I would turn 3 of them into a bean animal style drain with a gate valve to keep the drain quite and backup drain. Then I would reduce the 4th bulkhead to 1" for your return because you are going to be slowing down the flow with that drain setup.