The Dirt Commanders run very close to true size. It really depends on what air pressure you run in them. I'm running 9lb on my front and 7 on my rear for now, but with 8 ply carcasses, they are a very still tire and I am SURE I could lower that to 4-5psi like I had been running in my ITP 589s. My only issue with the 27's is I cannot seem to find a 2" bracket lift for my Wildcat, so I have to gain lift by oversized tires instead of sticking with my 27s plus a 2" body lift.
I know that the Bighorns run much smaller than advertised - so many guys run them these days, and the 27s measure out to about 25.75. Do your research on this, as some of them run well over an inch less than advertised. I mention the Bighorns simply because they are so well represented here in PA on our trail systems.