Custom Mats. !! MAX Custom Opening Size is 2 inches smaller than Mat Size. This will allow for a minimum 1 inch mat border on each side. The exception is for 4x6 and 5x7 mats where the largest Custom Opening is 1" less. Ex., 4x6 Mat, 3x5 max opening. 5x7 mat 4x6 max opening. This will allow for a minimum half inch mat border.