Just be sure that it can handle the flask, filled to 5L. I think it's more a weight concern with most stirplates. Plus, the size of the stirbar it can spin, in the starter, and still get a good vortex going. I have several different size, and shape, of stirbars right now. Depending on the size of the starter, determines which I use.
Something I've been doing more of late, though, is to do a two step starter. Basically, I use the yeastcalc.com to determine what size starters (1st and 2nd steps) to get the cell count. You can get the same cell count with two small starters than in a significantly larger starter. Such as two 1.5-2L starters giving the cell count that would take a 6L starter otherwise (all using stirplates too). With that, I have yet to actually use my 5L flask. I'm keeping it though, since there could be a time when I'll need to use it.