You should try pruning it around this time. It generally starts to produce flower buds in July for me; I guess that, up where you live, it may do so later in July or in August so pruning it now may help control its size and not affected next Spring's flower buds.

Some of the large oakleafs and paniculatas are plum too big for containers and should be planted on the ground so you do not have an additional garden chore to do. But when space is at a premium, pots and containers are the way to go.