Note that motherboards vary in appearance and available ports and connectors, and things like fan headers, M.2 connectors and the BIOS battery are found in different places depending on your motherboard.
Most motherboards come in one of three sizes:
ATX - Measures 12 x 9.625 inches. This is the standard and biggest. As such, it’s the best choice for features and expansion.
Micro-ATX - Measures 9.625 x 9.625 inches.
Mini-ITX - Measures 6.75 x 6.75 inches. The smallest option. Used in compact PCs.