I aggree I have a 125g and I have both a sump and a canister filter are attached to bulk heads, also if you plumb it right add a PVC bib and you have the easiest water change ever.
I'd do a 55 gallon or 75 gallon as a sump for a 200-300 gallon tank, if you get the right amount of flow(pump) and decent enough media that should do great.