lorax

Your Gran Nain has the potential to get very big over the summer if you water it well - here, where they're grown by Chiquita for commercial banana production, the plants go to about 15' of pseudostem, and leaves after that for another couple feet. They're very heavy plants.

If I were you, I'd get them all in the ground ASAP - that way they'll have time to establish themselves and grow before winter hits you. Your altitude will be less of a problem than you think, though. I have a garden at 10,000' and grow bananas in it (Plantains, DCs and Gran Nains seem to be the most altitude hardy.) However, if winter tends to strike hard in your area and with little warning, I'd do big pots for the first year, just to make bringing them in easier.