I'm wondering why this bamboo is looking so unhappy. Fortunately what you have is not a post-flowering Fargesia nitida, which would be on its way to a certain death, but I can't say exactly what you do have, I don't have my textbook to hand. I can say it is not the kind of bamboo most suited to pot culture, but that hasn't stopped many people before you. So I'm guessing it is looking rough because it has either been over-watered or under-watered. But it does look like it has a recently produced culm (on the right side of the plant) so I think its alive.

From the size of it, I think it is in an adequately sized pot - at the moment. So probably just do the obvious: refresh the compost if you can or top-dress it, water it very regularly (but don't let it get waterlogged - it should be able to drain from the bottom of the pot, and don't stand it in a deep puddle caused by a deep outer pot). And when it recovers (if it...) then start feeding it generously.

Later, probably in a couple of years, if it recovers and expands, it will no longer be in an adequately sized pot. What you haev to do is either repot it to a larger pot, or else divide it in two and return it to two pots of the same size. The best time to do this is Sept/Oct or March/April.