I have 3 different rotating mats. One is by Fiskars and the other 2 are Olfa. The Fiskars one is 13" on a side and is a light grey color. This is helpful if you are cutting dark fabrics, but I find the surface to be rather hard and stiff. The Olfa mats are 12" and 17". I use the 12" one the most. It is my favorite. These are very helpful if you are trimming a lot of blocks and need to turn your work to get all the sides. Depending on your work surface, you may have better luck than I do with the large one. I seem to have too much on my cutting table that interferes with the smooth movement of the top! I hope this helps.
Leslie