That totally depends on how many people you plan on letting in at one time. I mean 20 acres would be fun for 1-5 trucks, but you would probobly need 400-500+ if you plan on getting 50+ trucks at once in there.
Now i know people will argue with my figures, but tell the truth, do you want to be the 10th truck in line on a trail with a jeep with 3" of lift and 33's in front of you holding you up????? I think the more ground the better, ya know letter people do their own thing!!!