The area with a minimum circumference is a circle. Thus the maximum area with a circumference of 100 m is a circle with a diameter of 31.83 m and the value of the area is 795.775 mē

There doesn't exist a maximum length of the fence. If the area is a rectangle then you can calculate the length with respect to the width of the rectangle:"Also, if you want to enclose 100 square meters of land what is the maximum length of fence you can use?"

If w is becoming smaller and smaller l is growing more and more. In the end you need a fence of infinite length to enclose nothing.