I am sorry, I was talking about the refugium and you were meaning the overflow. Yes, the overflow size can be too big and should match your return pump. For example I was using a mag18, which is 1800gph, but after all the bends and height to the top of the tank was only pushing around 1200gph. What pump are you using for a return? Most overflows are either 600gph or 1200gph. If you are using a smaller return pump then you would want to go with the smaller overflow. I hope this helps you out a little more and sorry about the confusion.
I do not have anything at the moment. I am trying to figure out what would work for a 55g. So no pump or anything yet. Well since I have 2x-3x powerheads maybe I will go with a 600gph overflow.
The overflow rate should be equal or greater than the return pump rate but the tank will balance with different combinations. For example if the overflow will drain 600 gallons and the return pump will push 300 gallons, It will balance at 300. The overflow can ony drain the amount of water retuned to the tank. If the return pump out performs the overflow, the system theoretically could suck in air, but again the pump can only return the amount of water supplied by the overflow. I have a few different filter/overflow systems availible right now, if you would like to send me your contact info, I will give you a call.