Existing canes, once fully expanded for that season, will never grow any larger. If you have shoots currently expanding, you'll just have to wait and see what they do, since their performance will largely have been set by the root structure established over the previous season.

Now that you have it in the ground, it has the opportunity substantially to expand its root system, given sufficient food and water (and nature unaided can be quite adequate depending upon the location, though supplementary assistance is advised in other places). July-October is the key period. Adequately fed, you can expect it to send up ever larger culms each year, to a mature size in the region of 12-20 feet. Even a somewhat undernourished plant will proably achieve 10 feet. If you wish to restrict it to 7ft, you'll either have to starve it (but not so much that it declines), or (not aesthetically approved of) trim it.

Over the next few years, assuming you feed and water adequately , you'll get a larger clump, with larger culms, and the larger culms will have longer branches and more leaves. Be aware that it will have fewer leaves near the ground, and more higher up.

Then when it is ready, it will spread more widely, by sending out long runners. Phyllostachys bamboos nearly all run eventually, but how fast depends on climate and species. In much of the US, P nigra is generally pretty rampant because of their warm wet summers. In the UK's cooler and less generously damp climate, they are considered fairly well-behaved, often waiting up to a decade before sending out some fairly tentative runners. A well-fed one I have seen pictures of in mild North Kent was getting fairly aggressive fairly quickly. In contrast, some clumps in gardens near my office in central London, where they have to compete with mature shrubs and trees in a heavily shaded, dry shrub border, are sending up rather weedy shoots that will be smaller than existing culms.

Because of the eventually spreading tendency, if you have it close to the boundary of your property you may wish to insure against annoying your neighbour by installing a rhizome barrier.

If you really want a reliably 7ft, reliably non-spreading bamboo, I suggest you whip it out and plant a Fargesia murieliae "Simba" instead. Because it won't spread, (other than by gradual radial enlargement) you'll have to plant a clump once every 3-4 feet to get a hedge out of it.