Motherboard size for my Mini Tower Case? MicroATX or ATX?

ATX is 9.6 by 12 inches so it may fit, but micro-ATX should fit. If your case has at least 5 expansion slots, then ATX will fit. However Micro-ATX motherboards will fit in almost any ATX case because the screws are in the same place, and there will be a gap between the botom of the case and the motherboard. An ATX board will probably fit if it has more than 4 card slots in the back, but i would go with Micro-ATX to be safe.