Just returned from a vacation which included a week at West Yellowstone. Our troop included my wife and myself, two 5 year old grandchildren, my daughter and son-in-law all in our 39ft MH. Fantastic trip. We stayed at the KOA West Yellowstone, which is about 6 miles from the park entrance. Lovely RV park, and I chose it because they allow tents outside the MH. As luck would have it, it rained every evening, and we decided to all sleep inside, but it was still a good choice as it is a lovely RV park. We traveled with another couple who traveled in their rig. Therefore we rented a 15 passenger van from BIG SKY Rentals in West Yellowstone. This turned out to be a wonderful plan, as we could see everything and pull into all the turnouts without concern. They also rent smaller vans and cars. Very Nice people to deal with.
As for what size RV is best: Bigger RVs sleep people more comfortably. But a large RV just will not fit into the turnouts and pullouts. Many years ago, like 30 or 35, we traveled Yellowstone in a 28foot C, I remember that we could not fit into many of the turnouts back then...and the roads have not changed.
Hope this helps with your planning.
Have fun
