Flask size isn't really the problem here...it's magnet strength and the distance between them that matters. The closer the stir plate's magnets are together, the smaller the stir bar you can spin. The stronger the magnets are (or the closer you can get them to the stir bar), the larger the stir bar. There's a happy medium somewhere...
I can successfully spin everything from a 1/2" to a 2" stir bar in a full 2L flask with two 5/8" diameter rare earth disk magnets spaced roughly 1.5" apart on top of a computer cooling fan.