renting before buying

I have never owned an RV before and would appreciate some input on several questions. I am interested in a class B but I'm not sure how big I should go. Its my wife, myself, and 2 yr old. I've looked at a 19ft., 21 ft., and 24ft. My main concern is being able to maneuver the vehicle easily, is there a big difference in driving around a 19 ft, as opposed to a 24 footer? I was thinking about renting a 21 footer from adventure touring Rv rentals and seeing how this size is. I would like to try the 24 footer, but am affraid that parking, ect. could be a problem for me since we are going to be traveling in Colorado for the first time. Thanks