The natural form of hydrangea paniculata Kyushu is somewhat open, vase shaped, tall. Best pruned in early spring, blooms on new wood. I normally prune paniculatas in Spring as the growth buds just begin to swell, cutting back to a pair of those buds - how far back is really your choice. Pruning will result in fewer but larger flowers...which can be a good thing depending on the effect you are looking for. Downside is that a summer rain can weigh those larger flower down, sometimes to the point of breaking stems.

Thank you for the help.