I really like my 8.5 x 24.5, it's my most used one. Don't know if going to 12.5 would be nicer or more of a pain. I also have a 20.5 x 20.5 that I use for squaring up. I use the 8.5 to ruler fold my yardage (perfect size for my shelves) and then I can fit the ruler in without having to unfold a lot for cutting WOF.
I cut across the bottom, and not up and down. Because of that, I'm often balancing part of the ruler off the table with the 8.5" one, so that problem would be even worse with a 12.5 one. Maybe I should try to teach myself how to cut the other way.
I'm a huge fan of CG rulers, and find them worth every penny I've paid. Though I've gotten many of them at a discount at quilt shows.