end mill bit sizes
As with all things associated with board fabrication, the size of router bits used by the fab depends on their equipment and the thickness of the board.
In general, the preferred cutter size for routing is 3.175mm (0.125 inch) or 2.36mm (0.093 inch) diameter. Smaller bit sizes can and are used, but may cost more because the fab will have to slow the route speed to avoid bit breakage. The larger the bit you can get away with, the less the fab is likely to charge you.