A few words of advice. Lay the clump on some dirt and get an old axe and chop. This works well and saves time. Also. Be sure to do this in the shade and keep rootballs slightly moist when working on them.

I just meant a standard nursery pot...not something that might have a rough texture inside, like some large ceramic pots. As for size, it depends on the plant size now. Growth will be slower in the pot rather than the ground. The pot should be big enough for future growth without being so large that the plant looks dwarfed in it. What gallon size plant do you have or intend to get? One other thing: when I grow bamboo in pots, what I usually do is plant it in a utility pot and then place that pot inside a slightly larger decorative ceramic pot that will accommodate it. That way it can be attractive and you can use any shape ceramic pot to hide the plastic utility pot, and you don't have to worry about the bamboo being stuck inside an expensive container when it's re-potting time.

I think you could build long, narrow raised beds. How long an area do you want to cover? How tall do you want the bamboo to get? I assume your climate is similar to Seattle? Anything about your location that is "special"? (high winds, extra dry, etc.)

if your in zone 9 look at some of the bambusas, such as gracilis. You can put these in the ground with no worries of them spreading.