Hibiscus syriacus can be and is actually enhanced by hard pruning in early spring. Flowers are produced on current season's growth so no fear of losing them, although hard pruning generally results in fewer but larger and more showy flowers. You should be able to maintain it easily at the desired size.

Gardengal,
We have had ROS for years handed down from my father-in-law years ago. (he's REAL old :)
We've always trimed ours back in the fall after the last flowes have died and that's what I recommend to customers. You think it makes much difference from spring to fall?

It could. Generally, pruning back of deciduous shrubs (those that flower on new growth), especially severe pruning, is recommended while the plant is still dormant but just before budbreak - late winter/early spring. Depending on location and if winters are hard, you could get dieback, sometimes significant amounts. If the plant has already experienced substantial pruning before the dieback occurs, there may not be enough woody tissue left to protect it and it could be killed. Even in my mild climate, ROS often gets a lot of winter dieback and so pruning in late winter or early spring removes all of that dead wood as well as reduces size without fear of more damage from winter cold or late frosts. Most - if not all - pruning manuals will recommend spring pruning.

But, ROS are pretty tough plants - if it works for you, I'd not say to change your methods.