Have to agree with Jim. Dredging more is usually better. But, if you live in an area with limited or restricted stream or creak flows, you may not be able to use a dredge larger that 2.5" -3". Most that use dredges larger than 3" are submersed under water diving or semi diving. We live in the desert and have ability to use a 4" or our 2.5". In the 4" case, under water submersed in a moderate sized river in a deep hole. With the 2.5" waste deep or less in a seasonal creek bed. I can handle the 2.5 by myself no issues, the larger not so easy, not really safe, under water alone so don't do it alone.
Evaluate, budget, go for it. Have fun and be safe.