I buy my size and the waist fits fine. I don't make any adjustment for them being washed. I've had at least half a dozen of the washed duck pants.
Those are my measurements too, and that's the size I buy, and the pants always fit fine. I usually wash them first, so I can't say how they feel and fit prior to washing, but beyond that I haven't noticed any shrinkage, to speak of. I think they must run pretty true to size.