Rectangle
Area = Length * Width
Ellipse
Area = Pi * A * B
Where Pi = 3.14159..
A = semi-major axis
B = semi-minor axis
Definitions:
semi-major axis: The major axis of an ellipse is its longest diameter.
The semi-major axis is one half of the major axis, and thus runs from the center
to the edge of the ellipse.
For the special case of a circle, the semi-major axis is just the radius.
semi-minor axis: The minor axis of an ellipse is the longest line that runs perpendicular to the major axis.
The semi-minor axis is one half of the minor axis.