I have no opinion about the size, which I think will vary according to what you need to cut at any given moment, but I have to say that Pat Speth's video in which she demonstrates the use of a piece of thin foam shelf liner to keep rulers from slipping has been a real game changer for me. If you haven't tried this, do!
https://www.youtube.com/watch?v=4042cAxo5aY The only method that has worked as well as that for me, with any ruler at all, is coating the back with rubber cement, but that's so much more bother and looks icky after awhile because threads and lint stick to it. I don't attach the shelf liner to my rulers. I just keep a 12" X 4" piece of it on the cutting table and put it under any ruler as needed. It's a fantastic discovery. That shelf liner drove me crazy on shelves where it keeps dishes and cans from sliding when you push them, but I'm glad I kept a bit of it for the sewing room.