I have this boot, as well as Red Wing Beckman round toes, Iron Rangers, and 875 moc toes. I wear a 10D in an Allen Edmonds Park Avenue. My feet are on the narrow/low volume side for a D. I find that all the Red Wings I own fit pretty well in 9.5D. In the Rancourt Russells, which I picked up on sale recently, I got the 10D. They are by no means too long, so I certainly wouldn't go to a 9.5D. As described, they are quite roomy, in terms of volume and width both in the ball and heel. They work well for me in heavy socks.
I hope this helps.