The original gravity of the beer you brew determines how large of a starter you need to make. So the bigger the beer, the larger the starter.
You can make a starter with 1/2 cup DME and 4 cups of water and you'll have some headspace in the 1000mL flask. But if you want to step it up a few times (which larger beers will require) you're going to run out of room. 2000mL should be good for 5.5 gallon batches but if you plan on doing 10+ gallon batches I'd say get the 5000mL flask.
I'd suggest putting a drop of fermcap to help control the foam from the starter. It's also great when you boil because you don't have to worry about boiling over.
Below is a link to a calculator that will help you determine how large of a starter you need to make.
Mr Malty Pitching Rate Calculator