somewhere i found a calculator for sump size, I did some complex math and came up with a return area of 5 gallons for my tanks( a 75 and a 36) based on the return flow and height with the overflow baffles. I then added in what size and depth i needed my intake and I was at that time using a modded hob
skimmer coralife ssc65 (still using it just added a better pump), which left my refugium in the middle taking up about 5 gallons. My decision for having it be in the middle was I didnt want to divert any flow away from my tanks and i didn't want another pump just for my refugium. This changed with the addition of the new skimmer that will not fit in the first chamber,so i took out my refugium, and may add a remote refugium at a later date. I am using a 29 gallon and with the 5 gallon ato container and the hob
skimmer and the fans and the reactors its all pretty much full.