Generally, sump aize is limited by the space you have available under your tank. It will depend on cabinet size, since not all stands are equal. I use a 20long for my sump with good results. One thing to remember - unless you've designed several sumps, don't try to push as much flow as possible through it. I pretty much maxed out my overflow with my return pump at around 600-700gph, and I get a lot of microbubbles through my sump and into my tank. I'm redesigning it soon and going to look for closer to 300-400 gph through the sump, and make up the rest of the flow with powerheads in the tank.
Total flow in the tank will depend on what you want to do. With a fish-only system you could get away with 10x turnover. FOWLR you'll want 10-20x turnover, and reef as much as 20x+ turnover.