Bracelet S is made for wrists about 45mm by 30mm. Fits perfectly for my 6 years old.The inside is 48mm X 35mm, the back opening is 20mm wide and with height is 20mm.Consider printing with 100% infill for translucent materials to prevent infill lines to be visible.UPDATE: Updated the Bracelet S to be a bit wider on the opening (20mm instead of 17mm)Bracelect M inside dimensions 58mm X 44mm. Opening is 23.5mm and height is 25mmBracelect L inside dimensions 64.5mm X 49mm. Opening is 27.5mm and height is 30mm*dimensions are approcimated