It depends if you are trying to create a tornado like vortex or merely keep yeast in suspension and promote gas exchange. If it's the latter, then a 3" Teflon coated stirbar will suffice. I prefer the octahedral ones with the pivot rings like this one. You can find them online or from one of the science supply vendors on ebay.
Cold worked 300 series stainless and 400 series stainless are magnetic. Whether they are sufficiently so to perform as a stir bar would be a neat experiment.