There is a container forum, you should try there.

Kristy, I've seen good size ones in a 5-gal pot at the nursery. I would assume a 5-gal or larger would be fine.

Mexican bird of paradise will not live more than about a year in even the largest container. I put one in a huge pot, and it got root-bound in no time and then died. They make extremely invasive roots and require a large area for growing.

If you live in an area with frost, you might be able to handle one of those, but I would never put one in the ground.