enough place for a huge cpu-cooler, dustfilters and a smart cable-management-system is what all is about in the end.
have an eye for cooltek antiphon, nanoxia deep silence 3 and fractal design r4. they are "average priced" and absolutely worth their money.
i personally use a mATX Corsair 350D, i bought dustfilters with it and im totally fine. all depends on the look u want and the abilities. i have a highperformance-pc built on mATX, so size doesnt matter in that link. (i heff big penis doe)