Either that, or just find the brand and model of your board, and look it up on the web.
Either way, unless you go for a case only made for mini-ATX boards, you're fine.
I think you need to do a bit of research, friend. If you're buying a new case, then I'm assuming yer either transfering parts to the new case, or building a machine. Form factors for motherboards are simple and common knowledge which every PC user who tinkers with his/her stuff should know.