AMA minimum recommendations is for the overfly area to extend 1000' beyond the ends of the runway and 500' on the far side of the runway. Beyond that is recommended another 250' on each end and on the far side of the runway as a safety zone. Also 80' on the pilot side of the runway to a parking lot.

If you have a 50' x 400' runway, that gives a total area of 2900' x 870' which comes out to 4.25 x 14.5 acres or about 62 acres (to meet AMA guidelines).

Scott