I have a 3 litre flask , working on the basis that you can fit 2 litres in a 3litre flask but can't fit 3 litres in a 2 litre.
It does of course also depend on the size of your batches and the gravity, if you were making high gravity lagers and cool fermenting a 3 litre might not be big enough. But if that kind of thing is rare for you ( it is for me ) then you'd find another big vessel to use as a supplement on those rare occasions.
Make sure your stir plate though can cope with the flask, most can.
I haven't wished I had a smaller flask.