I've been looking on Ebay for tire chains for my riding mower, but I don't understand what the measurements mean. They have three measurements: a long measurement, which I assume is the length of the chain, and then two shorter measurements. I assume one is the width of the chain, but I don't know what the other one is. Also, the diameter of my tires at the widest part are 21". Would the chain length have to be the exact length, or would a slightly larger length work? Same question for width. I'd appreciate any help I can get with this. Sue D.