"Boxwoods can be cut back pretty dramatically and they'll re-grow nicely. ... "But because boxwoods are very prone to winter damage, you want to time their pruning—especially a hard pruning—carefully. The best time is that period where the end of winter meets the beginning of Spring, just before the new growth appears. Check
https://extension.umn.edu/planting-and-growing-guides/protecting-trees-and-shrubs-winter for more information.