The size of your starter will depent on your target OG for the stout. A good general rule in building you starter is 1 oz of DME per cup of water. So a 2 qt starter would call for 8oz. For starter sizes, check out Mr. Malty's pitching rate calculator.
The volume of starter (should be ~2qts) should affect 5gal+ of beer. If you are worried about it, crash cool your starter for a day or two, and as BeerPressure suggested, decant and pitch the slurry.