Yeah PCPP will tell you if hardware will conflict with each other and if there are any problems with compatibility among the PC parts. Now, as for the size of the case, it will depend on your other computer parts. For example, you can fit either an ATX or micro-ATX motherboard in a full or mid-ATX tower, so it works out either way, but some cards may be too big for small cases, so you may need to get a bigger, full ATX case. The size of your case mainly depends on your other computer components and whether they can all fit into a mid-ATX tower or a full ATX one, but it might also come down to personal preference too.