Just to add to my earlier reply, the footprint I mentioned on the Urchin is likely for the skimmer body itself, the pump would add to that (you probably saw a picture or two), but a cool thing about the Urchin is you can relocate the pump just about anyplace in the sump. The hose that it comes with doesn't have to be used, you can probably use other hose if you need to work some placement wizardy, but I'd avoid too many bends and lengthening of hose.
You say its going in a "skimmer chamber", if there is a way you could position the skimmer so the pump would sit in an adjacent part of the sump, then you'd be good to go right from the box.