What is best to determine the size skimmer you need..... based on Tanks volume or total volume (tank + sump)
Skimmer sizing is based on theoretical loads (amount of waste/DOC's in the tank from low to heavy). Generally, I suggest basing skimmer size on display volume as that is where the load (fish) is concentrated. Our sumps and refugiums don't contribute to the load but instead typically decrease load by removing organics or nutrients. While some systems have proportiately small sumps in relation to the display size, others may have relatively large sumps and refugiums relative to display sizing. As such, basing your skimmer choice on display size is typically the best way to go. This is especially true with skimmers that are properly rated. That proper rating however falls into a gray area. Many of the really cheap Chinese skimmers have very optimistic ratings where as skimmers like the Bubble Kings, Deltecs are conservatively rated and very efficient for their size and really shouldn't be oversized. Skimmers like Life Reef fall into the same category although they do have smaller neck diameters relative to the body size so oversizing one slightly won't result in as inconsistent operation.. Reef Octopus, RLSS, Aquamaxx, JNS, Bubble Magnus are typically relatively accurate on their sizing and should be sized based on the display volume like the others mentioned above for best consistency and performance.
Edit: One other thing I want to add is relative to loads and fish stocking. This is somewhat subject but a heavy load would typically be considered 1/2" of fully grown fish per gallon of display volume. This is kind of a general rule of thumb when it comes to stocking a salt water tank. This gets a little confusing since bigger fish like tangs, rabbit fish, large angles etc have much more girth not than smaller fish like wrasses, chromis, anthias etc. So calculating a truly heavy load can be a bit confusing. Larger fish contribute to the load much more so than smaller fish because of their appetite and the amount of waste they generate but the 1/2" per gallon is generally a good guideline for a truly heavy load.
Also keep in mind that while this is guideline, this has nothing to do with the tanks ability to keep up with that load. Biological surfaces (live rock, sand, bio balls, other media) for bacteria to propagate plays a large role in the tanks ability to convert and manage waste as does mechanical filtration such as filter socks, skimmers, along with other forms of nutrient reduction such as GFO, Carbon dosing, nitrate reactors, macro algae deep sand beds etc and regardless of those, the skimmer will play a key role in the removal of dissolved organics which would otherwise break down and become nutrients.
