There was a discussion of this a while back. You should be able to find the information you are looking for in it:
http://www.coasterbuzz.com/forum.aspx?mode=thread&TopicID=37963
I would venture 60-100 acres on average for the large parks anyway.
"I go out at 3 o' clock for a quart of milk and come home to my son treating his body like an amusement park!" - Estelle Costanza