Sump design help...

[Dale]

As noted it really depends on what you want to accomplish with the refugium.
If you want a place to breed cope/amphripods, place frags and some macro algae the design isn't that important (relatively speaking). Leaving room in the return section can help when it comes to servicing the pump or as a place for heaters etc.
If you want a fuge strictly for nutrient control then you should try to design a chamber that grows macro well.
I like a fuge that tumbles the macro. This exposes more of it to the light source and decreases dead water flow areas. I have found that a square chamber tumbles better than a rectangle so you might consider an 18" x 18" chamber. You can either use the flow from the baffles to create a weir effect to tumble the macro or install a small powerhead to assist. I've also found that an AC 70 creates a good weir effect if you can hang it on the side.
Just some thoughts.