It does depend on your flame size, something like a pocket rocket or primus spider a narrower pot of 15 cm does make most use of the flame. If you use a wide flame stove like a wisperlite or a trangia meths burner (alcohol is usually a biggerflame) around a 20 cm pot makes best use of the flame. This is of course discounting a heat exchange pot, what you do not want is flames licking up the side of the pot. You of course know that so I wonder why you ask? The depth of the pot is usually variable on how many people, so I'd go for 800ml each plus 200 air space. I like skillets, and so do professional chefs as they are versatile, easy to stir and quick to heat, but you can't always take a 40cm paella pan with you.