Now would be a good time. General rule (at least in my book) is to prune right after flowering but not after the month of June. That allows any new growth that is stimulated by the pruning to harden off before winter (although I'm not sure where you are). Although I'm no expert, maybe start at the perimeter and start pruning in towards the center. I wouldn't be overly aggressive with the pruning all in one year. It might take a couple of years to get it to the size you want.
