Answer: Pruning usually encourages new growth so unless your weather will remain above freezing for at least 6 weeks, it would be better to wait until next spring to prune your arborvitaes. (New growth won't be frost hardy and you'll end up pruning again in the spring to remove winter kill.) The general rule of thumb is to remove no more than one-third of the live material from a plant in any one pruning session. Removing more than that can shock the plant and send it into decline. I'd prune 4-5 feet off the plant next spring, then prune it down again the following spring.
If you alternate the length of the branches as you're pruning you'll reduce the size of the shrub without making it look as though it recently had a haircut. It takes a little more time to prune in this manner but you'll end up with a more natural looking shrub.
Best wishes with your landscape!
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