I use a 1" for mine and it is a good size. Now for my 5000L starter it is to small so I tried a 2" and my fan/magnet combo is not enough to spin it correctly. A lot has to do with the magnet power, fan speed and magnet spacing.
I use a 1 3/8" stir bar in my 2000mL flask and it does a fine job. I think brew-boy is right in that the magnet spacing/strength is critical. Took me a few tries to get it right...
I use that exact one you linked in my 5 L flask for the first time last night. Even on the lowest speed, it creates a vortex that extends from top to bottom with a 4 L starter in it. Like the others have said, there is a lot of variance between stir plate set-ups.