well that is a difficult one as evry year the root stock will get bigger and bigger. What ever size you have them in now they will in time fill the pot and then they will need to be moved up to a bigger pot. Sorry I'm not really answering the question.

Hi, well Sbg is quite right, they will get a lot bigger, I bought a Canna last year, it had 2 stems, this year it has 18, but they can get bigger if planted in the ground, lift them in autumn, and keep frost free, and fairly dry, in early spring, plant up in appropriately sized pots, and start watering, still keeping them frost free, when the new shoots start to show, you can divide them carefully and pot up the new plants, gradually harden them off, and plant out after the last frost in your area.
I emptied mine out of their pot, and put the root into an empty compost bag, and put it in the unheated pantry, no problem, Derek.

Thank you for your help Seaburngirl and Derekm I will buy some bigger pots, thanks a lot

They look fabulous, where did you get the seed please?

I got the seed from a friend who I think was given it by a gardener who worked a big estate and he took of them off plants that were finished for the season . I still have some seeds if you would like them .

Yes please, when do you sow ect?

Where do I send them Debs

Have you got your seeds yet Debs

Hi Roger, yes thanks I do have the seeds. I will let you know how I get on with them. Thank you