I found the rainbow gradient in PS to create a rainbow that was too large for reality.
After gathering some information from sites such as http://eo.ucar.edu/rainbows/ I created a rainbow that filled approximately 1/21 of the radius. I erred only slightly on the side of exaggeration.
After adding the rainbow to an image and applying gaussian blur, the size is increased a little more, but I think it should look pretty close to authentic.