Right click on the radius dimension and select "Display as Diameter"
If you looking to display the diameter of a shaft, then create the dimension in the parent view and using shift (this will move the dimension) OR ctrl (this will copy the dimension), drag the dimension to the detail view.
The best example that I have found on how to do this was through a video created by Javelin. The first URL is to their page with photos showing how to do this. The second URL is to their video actually walking you through the process in real time (the video link is also the last "image" on the page). The only limitation that I have found is that you cannot get foreshortened dimensions to features that are only on one side of a part. (i.e. A boss or protrusion that is on only one side of a cylindrical feature.) It will still give you a linear dimension that reflects the radius. I hope that this helps!