i think you misunderstood the size parameter of lights. There is no such thing as a shadowsize. Cycles, as opposed to blender internal for example, does not support light coming from a single point in space. So what it needs to do when using build in light types is to generate some kind of geometry with a defined size > 0. I guess it’s some kind of implicit sphere in most cases (except for the area light obviously).
So what the size parameter in fact controls is either the radius or the diameter of the light source.
So now recreating the same effect with mesh lights is obvious, as richard marklew pointed out, changing the size of the mesh.
edit hmm doesn’t seem to work with nodes o.O