Are you in a warmer location where this will be a more permanent planting, or are you in a colder location where this is a summer-only thing? Cannas are generally hardy from about Z7+. If the former, there's no such thing, Cannas never stop spreading, outward/sideways. Start as big as possible, and plan to divide every year or two. If the latter, as long as there's a few inches around the sides of your rhizomes when you plant, that should be fine until fall. A privacy screen effect would probably be best achieved by having multiple pots, placed in a row. 3-gallon buckets would probably work well.

Thanks for your response, I'm just gearing up for a morning of planting! I like your description of cannas like to grow - it's very helpful. I live in zone 5, so this is a summer screen. I have both 5 gallon buckets and some growers planters ... they are slightly larger than 3 gallons, but the plants I had in them last year had aphids and I'm worried about that. Any thoughts?

Happy to swap anecdotes. :+) I wouldn't worry about aphids. If you see them again, knock them off with water hose & fingers. If you remember later, show us a pic of your Cannas growing!