Designing a sump seems simple on the surface. Throw some baffles into a glass aquarium you have laying around or maybe even doing your own acrylic work. How hard can it be? Right? We'll you'll likely end up with something that will work, but wont be optimized for space and performance and may have some serious and dangerous drawbacks. The easy way to avoid this is to follow a few basic rules.
Most often when you are designing a sump you are working under limited space constraints. Whether you have a limited sized stand or just have to work with the glass tank you already own to convert to a sump, real estate matters. You want to make the most of it.
What if you need to replace your skimmer down the road? You'll need to make sure that a new one will fit. And you won't want to be limited in terms of options because your skimmer section is too small.
Refugiums are great. Grow some macro algae, harbor some interesting creatures, or implement your DSB in the sump. And of course just like your aquarium, bigger is better.
The return section. Either your return pump sits in this section or it is plumbed to it. This is the clean pristine bubble free water that you pump back to your display. If you refill this section manually due to evaporation, you'll want this section to be large as well.
All of these design needs seem to be in conflict - each section demanding as much space as possible. So to start with arbitrating these needs here's my solution to find the best equilibrium.
The first decision when designing an sump should be to decide whether to install an ATO (Automatic Top Off). As the water in the aquarium evaporates, the water level in the return section will drop. If you don't have an ATO, you'll need enough capacity in this section to keep your return pump from running dry due to evaporation. Most people's minimum limit to keep them sane and allow for a "regular job" for this is over a days worth of evaporation. That means toping off manually is a daily task.
If you can implement an ATO of some type, you'll save yourself a lot of maintenance, be able to take a vacation, reduce noise, salt spray, and micro bubbles, and most importantly gain a nice chunk of your sump back for other purposes. If you implement an ATO, the return section can be tiny. Just big enough for your return pump.
If you wait till later to implement an ATO your sump space will inevitably be negatively impacted.
The skimmer section doesn't need to be enormous - just big enough to give you some decent options about fitting a skimmer in there. Research protein skimmer footprints for your tank size. Use a resource like Marine Depot which details this kind of information. Pick several popular models that your would consider buying down the road. Make sure all of them will fit in there in addition to the one you'll be plopping in there when you have this thing built. Skimmer footprints don't vary that much, so this isn't unreasonable. Making the section any bigger than this is wasteful and takes away from the last section, the refugium.
If you're planning a refugium in the sump, there is no reason to make it as big as possible. Throw in a DSB (deep sand bed), rubble rock, some macro algae, and a grow light. You'll end up with a great refugium design and your tank will love you for it.
The last question about space is bubble traps. Bubble traps direct water downward in an attempt to filter micro bubbles from the water. The idea is that bubbles float. Slow downward moving water will lose it's bubbles as they float toward the surface.
I'm a firm believer that bubble traps are a waste of space and can be removed from a sump design. That is a pretty strong statement considering most sump designs you'll find on the web contain a bubble trap of some sort. I do need to caveate the statement for those of you who don't have an ATO, a bubble trap may be necessary just after the return section water fall. (yes more space delegated to the return section)
I used to have a bubble trap on my sump and then I found a design on Reef Central which did not include bubble traps. After converting my sump, the number of microbubbles in the sump was actually significanty reduced. Here is the basic idea.
Make all the decisions that you can to make the refugium as large as possible. Make the skimmer section just big enough to fit most skimmers sized for your tank. The return section should be just big enough to fit: a days worth of evap and a bubble trap, or if you're implementing an ATO, just the return pump. Everything else goes to the refugium section.