The answer will depend on the shape. There is no reason for assuming that the area is in the form of a square.
4 feet
Each side would be 208.7 feet long.
A square four feet long on each side will have an area of sixteen square feet.
Each side of a square acre is: 208.7 feet long.
Each side is 8 feet long !
One side of a 40-acre square is 1,320 feet.
An acre is 43,560 square feet, so sqrt(43560) = 208.71032557111303591192697393256 Which makes each side of an acre that is square approx. 209 feet.
The positive square root of 676 is equal to 26. Therefore, each side of the square is 26 feet long.
864 square feet
Hint: the perimeter of a square is the sum of its four sides, so each side is one-quarter of the perimeter...
There are 208.7 feet on each side of a square having an area of one acre.
The area of a square of a with 82 feet on each side is: 6,724 square feet.