The exception is for 4×6 and 5×7 mats where the largest Custom Opening is 1″ less. Ex., 4×6 Mat, 3×5 max opening. 5×7 mat 4×6 max opening. This will allow for a minimum half inch mat border. Custom Size Bath Mat . Custom Bath Mat, Personalized Photo Bath Mat, Text Bath Mat, Bathroom DOkayr, Custom Door Mat.