What AlanB said plus:
For dry fly dubbing with almost all the natural furs just pick out the guard hairs. Some naturals, like mole, are onlky 1/4" or so. For yarn it depends on the coarseness of the fibers. Softer less coarse yarn fibers can be longer, say 3/4 - 1". That's not a fast rule. For nymphs and coarser hair there's still no fast rule. Depends on the 'look' you want to achieve. For instance, if you want to get a 'seal type' look, you can use Aunt Lydia's Rug Yarn cut into 3/4" pieces. Put in a coffee grinder and just pulsate the machine for a second. Do this as many times as necessary to get the fibers to fluff but not melt.

Allan