I have 2, 4, and 6L flasks. Its the 6L that I use all the time. Using the larger flask is better for even small starters since it provides a very large surface area for oxygen transfer to the wort. I pump filtered air into the flask headspace to keep the oxygen content up and flush the CO2. I find there is no need for a stone on the air line in this case and that helps avoid foaming.
I do like Amanda's idea of using an alternative jar for starters. Clearly, with a 6L flask, it will be crying time if I have a mistake with the flask.