matx should fit, it just depends on what else you are trying to cram in there with it. If water cooling is not needed, it should fit. If you want to use multiple PCIe cards, you may have to enlarge it a little. Scale it to the size you need.
My junior case is approx 5 x 11 x 11.5 inches on the interior (13x28x29cm). Add some for the top/bottom handles on the exterior dimensions.