Check the motherboard manual. It will identify the size. If you can't find the manual, then check the manufacturer's website for dimensions or size standard.
If you know the model of your motherboard you can tell us/search about it
To know what your motherboard is
Download and install CPUZ and it will tell you what it is
You only will need a mATX case if you want a smaller case and your current motherboard is mATX. Any case that fits an ATX motherboard will also fit a mATX motherboard.