what to avoid
I have several, in many sizes, and they are all fine. Whatever you buy, most definitely avoid the generic ones sold at Joann's. The measurement markings are off by 1/4" by the time you are halfway across them! Possibly due to being printed while on a roller of some sort...stretches the surface? Stick to brand-names for best accuracy...Olfa or Fiskars, and maybe think about a light-colored one and a darker-colored one for visual help. Also, for use at classes, Omnigrid makes folding portable cutting mats/ironing surfaces in several sizes. I have two small ones that I take with me. Very portable, and keeps me sewing while others are lined up at an ironing board with a possibly icky iron.