IDE is not the size of the motherboard, it is the drive interface. The size would be ATX or M-ATXor AT or something to that effect. It does matter what size case you get, but if you have an ATX board an ATX case is all you need. You can get bigger, but you cant go small than a mid ATX case.
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.