mid-atx = 4 External 5.25" Drive Bays and 2 External 3.5" Drive Bays
micro-atx = 2 External 5.25" Drive Bays and 1 External 3.5" Drive Bays
there are other sizes, I'm just not familiar with the measurements or compacity,
if you have a micro atx motherboard, anything as big or bigger(atx of course) should fit ok...