We rented the Dollar minivan in January. There were 4 adults and 1 child. We could barely get all of us and the luggage in. The other 2 adults and child sat on the back seat and we had to fold down the middle seat and pile suitcases on it. There were at least 5 suitcases and a couple of carryones. I wouldn't even think of renting this size vehicle for that party size.
I had a group of 9 go a couple of years ago. We rented 2 full size cars and that seem to suite us. Plus gave the grown kids the flexibility to go where they wanted. With the ENTERTAIN code for dollar, I booked a full size car for base weekly amount of $140. this code worked for our Aug, Sep and Oct trips. IMO, I would never go with the HOPES that I would be upgraded. As an National EA member, I also book the size I want, never hoping that there will be a bigger car in EA. I think the worrying whether you're going to get the size vehicle you want would drive me crazy and put a damper on my excitment of the trip. It may work out, but it may not work out....not the way I would want to start my vacation.