Large Size Bag: 15.6" x 15.6"; The opening for the letters to go through: 13.26 x 3.51" | Large Size Bag: 18.7" x 15.6"; The opening for the letters to go through: 13.65" x 3.9" | Medium Size Bag: 13“ x 19"; The opening for the letters to go through: 10.92" x 1.56" |