I don't know if these will help, but you may like to see them anyway. Sadly I did not have a tape-measure with me at the time....
http://www.trainweb.org/railwest/temp/ring-arm-front.jpg
http://www.trainweb.org/railwest/temp/ring-arm-rear.jpg
From the limited evidence available, it is believed to have been one of the two arms at the exit from the Lower Yard at Templecombe Junction.