Praise the Sun and drink SunnyD from you very own Estus Flask!
Scale to your preference, it's about 6.5" tall, though I would print it a bit bigger, to resemble the scale of the in-game model.
Shouldn't need supports, made it so the inside doesn't overhang.
Might have thin walls if you print it very very small.
Flask V2: modified so it has less chance of having thin walls at smaller scales.