I can't seem to find how many sq feet an enclosure needs to be to keep a single adult box turtle. I plan to have it outdoors during the day and indoors at night.
I would say that a yard appx. 3'x10' might be big enough. Just remember that you have to have lots of plants, a waterer, a feeding station and maybe a couple hiding places. The more "things" you put in the yard, the smaller it gets. So go as big as you can.