all good advice, 1.Bigger is better 2. enough room for equipment 3. enough room for syphon back (although as long as you dont put your return deep into the tank there wont be much syphon back). I always keep my returns as high as possible to eliminate this threat. You can test it once you have the sump installed, just turn off all the equipment and watch the water rise in the sump. If it get to say within 2" of the top of the sump thats cuttn it close. I prefer to have like atleast 3" from top just incase. Also, i'd make just enough room for skimmer to fit and the biggest area to be the refugium, the return side doesnt have to be that big either. Just enough room for the pump.