You know GG - I've wondered about this (and how big is a bunch) and was searching around for an answer when you posted this. According to this page on Herbs, Spices, and Seasonings - a standard sprig is assumed to be 4-inches long, unless othwise noted in the recipe.
I know that if I buy "fresh" herbs from Albertson's
they all come in a clear plastic box about 4-inches long, but if I get a bunch of herbs from Central Market
they are tied together with a rubber band, have the stems in water to keep them fresh, and range anywhere from 4-6 inches long, generally longer than 4-inches.