Answer: In your area, there may be some winter damage each year so you may not have to worry about pruning for size specifically. I would suggest removing any winter damaged stems in the spring once you can see how much of the stem has been killed back, then possibly a bit more if necessary to reach that half way mark. After those stems have bloomed you could trim them back a bit if needed to control the size. You can also remove the flowers as they fade. I would not prune them back in the fall unless they are very tall, the reason I suggest this is that in my experience the older stems can help protect the lower part of the plant over the winter. You may in fact find that your plant is killed back nearly to the ground each winter. This is not as bad as it sounds because this plant is able to bloom on both old wood (in early summer) and on the new growth of the season (in late summer) so if winter damage is extreme you will still have some flowers later on. I hope this answers your question.
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