It can be done on relatively small lots. More depends on terrain and hard obstacles than absolute size. Bigger is usually better though
This is the smallest site I've run on and it worked fine for events with 40 entrants or less, one car on course at a time (out and back). Scale in the lower right corner, not sure how many acres.
Courses at this site will always be pretty short, 1st and 2nd gear courses of a more technical nature. With bigger sites you have more options: keep it tight and technical or wide open banzai sweepers and such. They're all fun to me.