You cannot inscribe a circle in a general rectangle. That is because for a rectangle that is not also a square, the circle will not be tangent to at least one of the sides. Consequently, there is no way to determine the area of the rectangle based on the radius of the circle unless the rectangle is, in fact, a square.
In the case of a circle with radius inscribed in a square, the sides of the square are congruent to a diameter of the circle. Hence the area of the square would be
My calculator said it, I believe it, that settles it