I think typically, a good rule of thumb would be a container that is in 3:2 proportion of top growth:container looks balanced, though ideally, I suppose the root mass is probably as large as what is above ground. Well draining soil mix is a must -- sand and small gravel mixed in, I think. Butterfly weed would look lovely in a concrete planter.
I seem to remember reading that Butterfly Weed doesn't like to be moved/have roots disturbed. That means at some point, the container-grown plants will start to decline from overcrowding, but you might not be able to save it by transplanting.... though perhaps you could do this in winter while the plant is dormant