[twindman]

If you want full hookups and are longer than 34 feet or so, you are out of luck in the park itself. I squeezed into a site several years ago at the Fishing Bridge campground when I had a 34 foot Montana. Barely could fit.
If you don't need full hookups, then I think maybe the Canyon rv park would have large spots.
If these don't work, the Grizzly RV Park in west Yellowstone is VERY nice.
You need at least a day or two for the upper loop in the park and the same in the lower loop. West yellowstone entrance is kind of in the middle so you can go to either the north or south loop.
Have fun.
p.s. On what jcurtis said about distance to WY or Cody - I think Cody may be 70-80 slow miles to the entrance, whereas WY is about 15-20 miles of a straight shot to the loops.