Obviously you can tie them as long as you wish, but my usual target length is about 6". For that length, I try to keep the hook about 4 to 4.5 inches behind the hook eye. This makes a very nice general prawn pattern on a standard hook or tube or waddington without a long trailer if you limit the tail section to 1 to 1.5 inches and make it a little sparser . The orange one actually has the hook at about 5 inches back. That combined with the lead eyes is why that one hasn't been fished
With a fly that big under the right conditions, I don't feel a need to dredge.