I go a little longer than a shank length for most of my spinners, but the tail has to be very sparce.
However, for smaller spinners of size 20 and smaller (i.e. tricos and small BWOs) I do not use tails and have seen no loss of effectiveness. The nice sparce tails needed for spinners are very hard for me to tie and I try to avoid them if at all possible. Years ago there was a big debate whether or not stiff tails pushed the fly away from a trout's mouth and many guys promoted soft hackle tails over stiff ones light boar's bristle, moose mane, plastic etc. Then I started noticing that for many tricos on the water the tails got plastered to the wings and didn't really show up. That pushed me to doing away with the tails and I haven't regretted it - although spinners look a lot sweeter with nice tails.
On the opposite side, Jim Serio's Big Spinner pattern uses a bunch of Antron yarn for a tail and works just fine on those tricky West Branch of the Delaware browns. Maybe the trout don't might a bushy tail on a spinner as much as we do.
