I got this one on Overstock, and I love it, but I probably wouldn't have bought it if I had not had a lot of Club O bonus bucks to offset the cost:
http://www.overstock.com/Crafts-Sewi...I&searchidx=41
Before that I got along for a great many years with an Olfa about half that size, but it finally wore out. The 24X36 is about the largest size that makes sense to me. It fits my folding table nicely, and my arms couldn't reach any farther. I like the gray color.
I also have June Tailor Cut and Press 1 that flips to an ironing surface and I like that a lot for trimming because it rotates easily and can sit on a small table next to me when I'm sewing. Don't remember where I bought it - probably Jo-ann's.