For example, if your main display tank is 90 gallons and your sump holds 30 gallons, then you will need a protein skimmer that is rated for at least 120 gallons.
If the protein skimmer is rated for tanks of 60-90 gallons. 90 gallons being for heavy bioload and 60 gallons for light bioload. So it's important to note the manufacturer's recommendation for the tank size rating.
If you can afford a higher rated skimmer and your space permits, then you should consider buying a skimmer that is rated slightly higher than your tank requires. You might increase your bioload as time goes on so it wouldn’t hurt to have a larger rated skimmer providing it is not way oversized for the tank.