The binding frame size of A3 paper is 25mm in width, and the other three sides are 5mm. For the binding frame size of A3 paper, the width of the binding edge (that is, the long edge) is 25mm, while the width of the other three edges (that is, the short edge and the two opposite edges) is 5mm. The purpose of the design is to provide enough space to bind the paper and keep it flat.