It is best to get one that will turn over the your tank water volume at the same rate as your sump so that you are processing all the water thru your skimmer.
Bingo.
If you pay attention to guidelines from the more established skimmer makers, it works out this way in most cases.
Skimmer and sump flow roughly match at around 4x the display tank's volume. (Count livestock tanks, but not sump volume.)
It's a solid rule of thumb. (Is that an oxymoron? )