A M-ATX motherboard will fit into a larger full ATX case if you want.
What are the parts that will go inside?
My criteria for a good case would include at least two front 120/140mm front intakes that have a filter.
The filter keeps your parts clean.
160mm available for a good air cooler.
A cheap case is ok, but bust your budget for a case you really love.
It will be with you for a long time.
I use a Silverstone TJ-07E which is excellent but a bit pricey.
I picked it because my available space limits the case depth to 16"