From what I’ve seen using Google and Live maps, the property will be split almost equally between 2 parking lots (public and employee) and the park proper, the vehicles take up just that little bit extra. I’m disregarding some things, such as paths and roads.
I don’t think that there is any physical minimum size that a park must be. Although, you would be really limiting yourself on just a couple acres of land. (Anyone want to ride their only ride again? Hey, we won’t lose our vehicle in this parking lot.)