go for the largest sized board you can afford and that will fit your work surface. I have the 36x24 (or thereabouts) size cutting mat and I love it. If I had a larger surface, I'd have a larger mat. I have smaller mats that are rarely used. My 8x12 I use in other locations, like on the back patio during the summer, when cutting scraps into specific sizes. Other than that, it's my large mat only.
Laurie