I have some in 5 gallon nursery pots that are flowering. Just make sure you have holes in the bottom and use a light fast draining potting soil so they don't rot. I have flowered them in 3 gal size but once they get large and the rhizomes start multiplying, they will start busting out the sides lol. Even in the large pots I have to divide them every few yrs and replant them so they don't get too crowded.

Plants do best in the largest pots you think are reasonable. How large of a pot you can or should use (upper limit) is not determined by the size of the plant or root mass, nor by the size of the pot it's currently in. It's determined primarily by soil choice. With the appropriate soil choice, there is no such thing as over-potting.

BTW - inquiring about what I just said and gaining an understanding of how water behaves in container media will probably do more to increase your proficiency as a container gardener than any other single factor.

Al