Either should work, a mile is 5280 ft, a square mile is 27,878,400 sq ft.
1/3 of a mile is 1760 ft. = 44.7 meters which we round up to 50 m.
1/3 of a sq mile is 9,292,800 sq ft or (3,048 ft X 3,048 ft.)
Over the years of discussion, we have sort of standardized the suggested base at 50 meters unless otherwise noted. We just don’t tell anyone, so forget that you read this.