This is quite simple, actually. ATX is the standard size motherboard. This means that as long as you have an ATX case (which we could determine if you'd post the make/model), you can guarantee with certainty that an ATX motherboard will fit in the case.
By the way, the internal structure and outer aesthetics differ from case to case, but the tray where the mobo sits will always have the same screw hole pattern.
Don't just take my word for it though, remove your old motherboard (if you haven't already), and take a look at the screw placement. Notice how the holes align perfectly. This is an ATX-based universal design to ensure that all ATX mobos fit into ATX cases.