I follow the 3x's total water volume rule of thumb. I have a 55g and use this pump to feed my skimmer:
Mini-Jet Pumps Model 606
High performance submersible aquarium pump Multipurpose adjustable pump perfect for freshwater or marine use Pump motor's self-protected against overheating These high performance miniature pumps are ideal for providing circulation in aquariums and terrariums, powering small fountains, and creating water currents in reef aquariums. Since they're completely submersible, they're ideal for small compartments like sumps or reservoirs. Adjustable flow rate for versatile applications. 1-1/2" x 2-3/8" x 2-1/4" high. UL-Listed. 1/2" outlet.
Aquarium Water Pumps & Powerheads: UL-Listed Mini•Jet Pumps