In reply to mypyrex:
It sometimes varies a bit between brands of clippers, but Wahl (what most barbers use) number the grades of the detachable combs in 1/8" increments.
So a grade 1 is 1/8", 2 is 1/4", 3 is 3/8" and so on.
Longer ones do exist, but the longest that's much used is a grade 8, which cuts a length of 1 inch.
Grades 5 and 6 are exceptions to the rule. They can be straight 5/8" and 3/4" lengths but in most sets they're special tapered combs, one shorter to the left the other to the right (but I can't remember which is which).