That is easy to tell. The width of the back IS NOT the cutting width. Every JD sales guide explained how to tell the width of a disk. Here it is:
You count the number of blades on the front gang. Multiply that times the disk blade spacing. Then add the disk blade spacing one more time. That is the true cutting width.
So a 18 blade disk with a 7 inch spacing would be
18 x 7 = 126 inches Plus 7 = 133 inches
133 divided by 12 = 11.08 so it would be called an 11 foot disk.