Physical Dimensions
Besides the specs and form factors, the physical dimensions are also important factors in selecting a compatible power supply. Here is an outline of the physical dimensions of most standard power supplies:
ATX: 6x3.5x5.5", HxWxD. Most common. Uses 4 mounting screws.
Mini-ATX: 5x3.5x5", HxWxD. Rare size. Uses 4 mounting screws. Can be used in a regular ATX case, but often not the other way around.
MicroATX: 5x3x4", HxWxD. Use 3 mounting screws. Not interchangeable with ATX or miniATX.
Flex ATX: Even smaller than Micro ATX. Various sizes according to case specs; often not interchangeable.
Use the data above to determine if a particular power supply would fit your case.