I can't speak from personal experience, but I have a friend who has twin pots of paeonies on either side of her door, and they seem very happy. I believe they don't much care for being disturbed, so something large to start with saves their having to be potted on. Hers look quite crowded, and must be about 10 years old now, but flower profusely. I do know she protects the pots in winter, wrapping them in sacking against the cold, but I think paeonies are reasonably robust anyway.

as big as possible and dont let them dry out too much.