Any pot that fits the tubers, it's OK if it is tight, and you only need a few inches of soil. You can always transplant to bigger pots if needed.

I use a cardboard box lined with a garbage bag, and I fill that in with peat and shredded cedar along with the tubers, just be careful not to overwater because there is no drainage that way.

As soon as there are a few leaves, it is safe to bring them outside even close to freezing temps, but they won't grow as quickly as indoors with warmth.

I've had mine outside for 2 weeks already, and they've already experienced 34F several times without any problems, sunburn is the bigger threat, they should be gradually introduced to sunshine. I have about 30 clumps of various species, they are all doing well.

Try to get the biggest pot possible.

I put mine in 5 gallon pots. They did fine, but because of the speed with which they reproduced, the growth of new shoots was being hindered by the lack of space. In your zone I wouldn't expect them to grow too fast, at least until summer.

hope this helps.

Do not use to small of a pot if they are plastic. I am in zone 5 and they grow well enough to blow out the pot every year if plastic is used.

I start mine in 6" pots because I have so many and not a lot of room indoors. They get pretty potbound by May, slows down their growth, but I don't have room for bigger pots, so they just have to deal with it until they go in the ground outdoors, at which point I feed heavily and they take off.

I wouldn't start them too big because the soil will stay too wet and they are likely to rot. Better to start small and move to bigger pots when they are ready. Some sprout right away, some take a long time, so it's hard to predict when that will be.