Difference Between ATXs

Solution
There's no such thing as Mini ATX, you presumably mean Mini ITX.

Respectively: big, medium, and small, and that depends on whether you want a big, medium or small PC.


Generally the ATX motherboards, because theyre bigger, have more room for more PCIe slots. mATX will usually have a couple less, and mITX even fewer (usually just 2).
 

See above, there is actually a Mini-ATX, but it is fairly uncommon. I believe the OP actually meant Mini-ITX as I think others have assumed as well.