The sun lamp Size property in Cycles and Blender Internal is currently shared with point, spot and area lights. However as these lamps are rendered as if infinitely far away, it makes no sense to specify them in meters.
Rather you should be able to specify an angle instead. For example the real sun as seen from earth has an angular diameter of about 0.526° to 0.545°.
Implementation of this involves:
* Add a new sun_size property and give it a default value roughly the size of the real sun.
* Change the Cycles and Blender Internal code to use an angle (figure out conversion formula, this requires some math skills).
* Add a version patch in do_versions to convert the existing lamp size to an angle for backwards compatibility.