My rotating cutting mat is a Fiskers 13 inch. There are 13 inches marked on the top and it has a border around that of about one half inch. I don't know why anyone would buy one smaller than that. You can trim a 12.5 inch block on it, and it can be used for smaller blocks, too.
These mats are not meant to be used to make long cuts. Use your regular flat mats for that. But they work slick for trimming any size block from 12.5 on down.
My only problem with it is: I need to keep enough room around it to turn it around. Sometimes my area is so "junky" that I have to clear a few things away in order to use it.