Motherboards are usually ATX or ITX or micro-ATX, cases usually mention their "size" so to speak.
Most cases can fit normal ATX motherboard.
If in doubt, mentioning what case you have and what motherboard you are thinking of, will quite likely get someone to confirm it.
Bigger issue with smaller cases is usually if the graphics card can fit in, some of them are quite long and might not fit in case as easily.