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
A square four feet long on each side will have an area of sixteen square feet.
Each side would be 208.7 feet long.
Each side of a square acre is: 208.7 feet long.
Each side is 8 feet long !
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.
One side of a 40-acre square is 1,320 feet.
The positive square root of 676 is equal to 26. Therefore, each side of the square is 26 feet long.
Hint: the perimeter of a square is the sum of its four sides, so each side is one-quarter of the perimeter...
864 square feet
The area of a square of a with 82 feet on each side is: 6,724 square feet.
The length of each side of a square with an area of 225 square feet is: 15 feet.