I built a regular stir plate from a fan and large rare earth magnets. I can spin 5+ liters with a BIG vortex. Granted, I have a stirbar that is something like 1 1/2 inches long or so. The longer stir bar will help you spin more fluid.
That being said... you only need a small dimple in the top of the fluid to bring in O2 for the yeast and I run a much smaller dimple than the plate is capable when actually flocking my yeast.