to determine what ATX type\size of board fits just measure it:
ATX = 12 × 9.6 in
EATX = 12 × 13 in
mATX = 9.6 × 9.6 in
mini ITX = 6.7 × 6.7 in
but you will need to know more to ensure a compatible board.
like chipset, standoff mounting location, PSU power connector type.