My setup is small so Im not sure this will help or even apply to your build. I had mine made at Fujimoto. I needed a custom size tank made, I didnt want to drop a lot of cash, and I dont care for acrylic.
Because the sump is small I started with figuring out what room my equipment needed first. My skimmer needs to be in 8 to 9 of water and has an acrylic base footprint of 8 x 13. I had the skimmer baffle cut to 9 inches. My return pump is something like 3.5 wide. I gave it 5 so it had some breathing room and left the remaining space for the refugium. I easily have room for a reactor in the return pump chamber later down the road.
I dont know how much room you have but as always I would give yourself a little more room then you need if you can. More water volume is best but what is most important to remember in your design is that you have enough room in the sump for any water siphoned back in case of lose of power. You shouldnt only count on anti-siphons/check valves to save your floor.
Never mind the egg crate in the photo, Im in the middle of working on another idea. Im sure the other guys have more to add on the subject. Good luck with your built.
eace: