Corvair, I would reserve the vehicle that best suits your needs and hope for something better. I would not reserve something smaller than you need and count on an upgrade. You may, and probably will, end up disapointed. Plus, if you get to the car and it is too small and you need to go up a car class at the counter they will charge you a premium for it.
I always follow Murphy's law when it comes to things like this. The best rule of thumb is to plan on getting what you reserved and be happy if you get more.
Incidentally, I have been renting cars on every trip I have gone to Disney over the last 20+ years and I cannot recall one time I was offered a free upgrade to something better. Now, when I was in Vegas 2 years ago, I reserved a mid-size and they wanted to give me and my wife a Mini-Van. Bearing in mind this was our first trip alone without my son since he was born, I did not want a mini-van, we drive one at home. We ended up talking with the agents and they agreed to give us a PT Cruiser, so it worked out ok...