I went big and got
this one (24" x 17"). It is fabulous for getting a bunch of strips cut accurately and fast, and then sub-cutting into smaller units. It has a non-slip coating on the back, and I haven't problems with it slipping. I love being able to set it up once and get a whole bunch of cuts done before anything needs to moved, unlike the cut-move-cut-move process with a plain ruler. (I bought it in 2019; back then the price was $50.)