Smallest footprint is prolly a Tunze 9010/9011 skimmer (4.4" x 4.9") , but I think it actually takes up more space (like 4.4" x 7" ?) with the mounting bracket or in-sump outlet pipe installed. New they are expensive, but you can sometimes find them used for <$200.
I'd get an external skimmer if you have the space outside your sump. There seem to be a lot of used H&S A150-F2000/1 skimmers for sale around $300-$350.