the most common sizes are 4.1x9.5 or 3.5x9.5 depending on the ski and binding used. normally if the ski doesn't say anything, default and use a 4.1x9.5 which was the case for mounting my invaders with look/rossi bindings. contrary to that my BC's needed a 3.5x9.5 but its was written on the ski. BUT CHECK and hold the bit up next to the side wall to make sure you won't drill to deep. also screw in one of the binding screws you're going to use through one of bindings (doesn't matter if its toe or heel) just so its flush on top and put the bit up next to the remainder coming out the bottom to make sure you're using the right length drill bit.(if you have lifters plates make sure they are on when you check the remainder) if the screw is just a wee little bit longer than the drill bit its ok. i hope this helps... good luck.