You want the biggest sump you can get, because a bigger sump = more water volume, more water volume = more water stability. However, it doesn't really have to be of a certain size. It could even be as small as 5 or 10 gallons.
Judging by what I've read, the coralife super skimmer is the best bang for your buck. Usually, you want a skimmer rated for twice the size of your aquarium, so the one sized for 220 gallons should do.
Also, I'm moving this thread to marine products & DIY.