I bought my Fargesia rufa in a #5 pot (about 3.2 gallons). It was ready to burst the pot and had completely fill the surface with growth. I divided it into 3 sections: 2 two-fifths and 1 one-fifth. So the two-fifth sections were around 6 inches diameter. They did (and are doing) fine. Any section with less than eight culms died. Not sure how my climate (Minnesota) factors into this.
I always have some dieback as winter turns to spring, and some years I'll cut all the culms down and let them completely regrow. Doesn't seem to hurt anything, but my plants never get more than 4ft in zone 4, so I don't have that much to lose.
