I like the 10" for bigger pattern fabric. Also makes a larger quilt with fewer squares (shocking, I know). 5" is nice and quick. I also like the 2.5". What's cool about those 3 is that the square leftover from cutting with the 10" template leaves squares that are 5" and work with that ruler. The leftover from the 5" gives you 2.5" squares which works with the tiny one. Which really makes it into a no-waste ruler. I haven't used the 4th size (3.5" maybe? ).