The greatest area of a polygon with a given perimeter is that of a regular polygon. A regular rectangle is one that has all sides the same length -- a square. The side length of a square with 100 ft perimeter is 25 ft. The area of a square with such a side length is
A = (25 ft)² = 625 ft²
The maximum possible area is 625 ft².
__
In the attached, x is the length of one side. The area versus side length is plotted. The maximum is seen to be 625 ft² for a side length of 25 ft.