Lots of questions... I remember when I wondered all that stuff. First of all, when you say sump, it sounds like you want a sump/refugium. The size all depends on how much room you have for one. Something like a 20g or 29g tank would be perfect.
As for filters... you don't really need filters. If you are doing a refugium, use a DSB & some chaeto. I built a bubble tower in the corner in which my water drains into the sump thru. In this bubble tower I put a bag of carbon and a bag of phosguard. Some will scream that you need a phosban reactor to efficiently run carbon and phosphate remover, but you don't. Water runs thru chaeto just as easily as it does carbon.
You'll also need some baffles to create a bubble trap so that micro bubbles do not make it back into your DT.