My family has used Dollar twice..once in Orlando and once in Denver. In Orlando last Dec we paid for a compact and were upgraded to a van. We once owned a van so my dad had no problem driving in but the couple next to us got the same thing and were not happy with the choice (this was around 10pm). In Denver, we also paid for a compact and were given an SUV (comparable to a Honda CRV) which we own so once again it was not an issue and it was great to store our luggage!
Like the post above, I would suggest signing up for Dollar Express if you go with them. It was very simple at Orlando. They checked us in (inside the airport), we walked across to where the cars are, checked in at a another window, inspected our car, and that was it. We did report some dents on the car in Orlando but it was no big deal. Make sure you have cash to pay the tolls on 417...they also might have a travelers deal for the Sun Pass but I'm not sure??