You need to know the different form factors that motherboards have. There are three major form factors on the market today. Standard (or full) ATX, Micro-ATX, and Mini-ITX.
You can typically determine which is which by the number of expansion slots each motherboard has.
ATX = 7 or 8 expansion slots
Micro-ATX = 4 expansion slots
Mini-ITX = 1 or 2 expansion slots
Computer cases are designed to match these form factor motherboards. A computer case's specifications will typically state which form factor motherboard(s) it will accept. For instance, some cases will show that it will accept both ATX and Micro-ATX motherboards.
While physical sizes may vary slightly, the number and location of the motherboard's mounting screw holes will identify it's industry standard form factor.
-Wolf sends