Welcome Pike.
I recently wondered the same thing as you did. As the lights used in Sweet Home 3D are SphereLight instances and that an ellipse is mathematically some kind of an elongated sphere, it shouldn't be so difficult to create an oval light instead that would match the bounding box specified by WxHxD. By the way, it would be nicer than db4tech's idea who suggested to lock proportion check box for lights, to ensure the user can only create sphere lights. [;)]