I hesitate to use a specific measurement when describing a groom, because different sized dogs require different measurements. For instance, 2" on my giant boy's tail looks great, but many smaller poodles don't even have two inches of tail!
My basic rule of thumb is that the underside of the tail should be shaved as far as the bottom of the anus (when the dog's tail is down). Ring it around the top so everything's level, and it usually looks about right.
Some adjustment will have to be made for each dog, since many are incorrectly docked. If I get a dog with a very short tail, I do shave a small ring to define the tail. Otherwise you end up with a bunny butt! On dogs with short tails or low tailsets, you can shave an inverted "v" at the top of the tail to create the illusion of a longer, higher set tail. Be careful, though-- clipping the v too far up won't look good at all.