Hey Steve,
The sunlight is a infinite light, which means that only the direction is important.
In the details tab please check the Contrast slider for the effect you are after. It is a point light with the distance to Earth the real size isn’t really important. Point-lights creates always a sharp light, as long as the medium in which it works is not foggy, which the “space” normally doesn’t provide.
I haven’t checked the intensity, but try this: Set the render to 32bit/c float Open EXR for example, then in the picture viewer enable the Filter and set the white point to 20. With the right side of the curve set the “super whites” so they roll out a little bit more, which means the curve starts very early to be nearly flat. You might see details again. Anyway if 1000 or 200 units is the right value depents highly on the settings of your material. The Specular high lights might “burn” very hot easily with such high settings. I normally like to suggest to set the diffuse channel to a lower value. For the Atmosphere it might be higher than for standard objects, but give it a shot.
Falloff in space and for “parallel” light. If we consider the “inverse square rule” for light than the sun light will have a quarter of its intensity after we double the distance of Earth to sun. Not really something that would apply to this scene at all, even in space. If we consider the light beams that reaches earth as parallel, and the space as a quasi vacuum, then there is not really any falloff as it is described by the inverse square rule.
The simplest way to find a value for the light in such extreme situations is given to set the initial (too bright value) to half, if to bright in half again, etc. This works in log2 steps, exactly like a photographer does it. Having shot the moon and the sun, I know the huge dynamic between both. The more lazy way is to set the project to 100 frames and animate the intensity from zero to the max value. Key interpolation to linear! Render during lunch, and the little clip show you precisely how much % your light needs to have from the initial max value.
Perhaps Chris pops in and explains a little bit his thought process behind the set up and how it works in an cm environment. (You can switch scale for a project, but I guess that is nothing new to you, just writing for the forum).
All the best
Sassi
