I agree that Lucky Bamboo do not mind having their roots tightly packed together. I would not go so far as to say that they do not like extra space for root growth. These plants grow naturally below the canopy in rain forests with diffused light.

As far as width, I suggest a container that has no more than 1-2 inches space around the stalks. Roots should be 3 inches or so below the soil surface and you want room for drainage so a pot 6" tall seems about right.

For a 6" pot, pack about 2 inches of soil at the bottom of the pot. Place your plant in with the root system on top of this packed soil. Do not insert the roots into soil. This will make the roots point up and roots want to point down. Then pack 3-4 inches of soil over the root to just below the top of the pot. Lucky Bamboo like tight soil.

Some people like a layer of rocks near the bottom of the pot to help drain away water from the roots. If your pot has drainage holes this should not be neccesary but it is your call. Lucky Bamboo like damp soil but not saturated.

Hope this helps.