LEaves generally last one year to 18 months. Then the first season leaves will be shed. New leaves will appear from branches developing on the culms. Each year the branches will add another level of branching, and the leaves will become smaller with each level of branching. I do not recommend defoliation to reduce leaf size, let it happen naturally. Or if you do remove old leaves, wait until after the spring flush of secondary branching has begun.
Think of your tray planting as a ''forest'' on a connected root. Reduce the number of culms to an attractive number and placement. Keep at least 3 culms, cut out below the soil line any poorly placed, too short or too tall culms. Often, once you have a nice arrangement of culms with their second or third level of branching so leaves are small, new culms are removed for a couple years, to keep the nice image. Eventually a flush of new culms do need to be retained as the old culms should be removed about every 5 to 10 years, I get rid of old culms at about 6 years for my Phyllostachys plantings.