just learning said:
Thanks for all help! I'm still a little fuzzy on something- I've only recently started doing serious router work, and have very little experience with bushings. What exactly are the different sizes used for?
The simple answer is that you want a bushing a little larger than your router bit. So, for instance, if your router bit is 1/2 inch wide, you could use a 5/8 inch or 14mm or so bit. You can avoid some math by having your bushings and your bits in the same measurement system, but if they are different, its not a big problem.
A somewhat less simple answer is that sometimes you need to know the offset between the edge of the cutter on the bit and the outside edge of the bushing. For instance, say you want to build a jig to cut a slot 3 inches long. Your jig slot would need to be the exact width of the outside diameter of the bushing so it will ride smoothly in the jig slot. The jig slot will need to be longer than 3 inches to accommodate the extra distance between the edge of the cutter and the outside edge of the bushing, actually the jog slot needs to be 3 inches plus twice the offset (once for each end). Way easier if everything is in the same measuring system.
Hope this helps.