Bio-load is based on the type of tank you plan to create. A predator tank, which will be heavily feed, will have a higher bio-load than a tank with a couple reef safe fish and corals. Additionally, the more fish (even reef safe), the higher the bio-load will be.
Higher bio-loads require more skimming. On the other hand, if you just have a few fish and coral, skimming too much can strip out needed nutrients from the water. Thus the reason the bio-load of the tank is one factor in deciding what type of skimmer to buy. Some people buy a more productive skimmer and then dial it down, which meets the needs of their current tank, but also provides the capability to add more bio-load down the road.
Height comes into play as the skimmer should be placed in the water level recommended by the mfg. For example, I needed to raise my skimmer as the sump water level was too deep. This became problematic, as now I have only an inch above the skimmer before it hits the bottom of the tank; this makes it tough to remove the cap for cleaning. In hindsight, I should have bought a shorter skimmer.