OP, in reading some of your previous posts it sounds like you are not going FT, but probably half time so do not worry about a longer size. Get the biggest one your truck can handle as you are no longer a weekend camper. Sure longer is harder, but on Monday I always find a site big enough for my 42' usually in USFS CGs that do not take reservations. Forget it on Friday as a 22' camper will be in a 40' site even though the CG info says 32' max. With a FW you are higher than me so those rocks at the end of the site will be less of a problem for you making that 32' site big enough for a 37'.
The sites for volunteers are bigger and further apart than what the rest of us get to choose from if you decide to do this. In Yellowstone my 42' becomes a 40' as that's the max for some CGs. The best deal in W Yellowstone is Baker's Hole USFS a couple miles north of town. With the old farts pass sites with electric are less than $20 and make the landscaping in town look sorry. Google those in town and then Baker's Hole. My favorite USFS CG in the Tetons says 24' max BUT there is one site big enough for my rig and the car. I like this
site for Public CGs as you can zoom in and look plus you can turn on the topo feature to see the elevation and how cool it will be so no need for AC.