Answer: I would suggest a larger container such as a half barrel size. Wood lasts for years, some of the fiberglass and plastic containers may also last a long time. You would need to check with the manufacturer to see if it is intended for year round use in a severe winter climate with extensive freezing weather (not just the occasional frost). Most nurseries and garden centers offer a selection.
A larger container will offer more room for the plants' roots. The larger soil volume will naturally stay moister and cooler longer in summer and will provide better insulation for the roots in winter. It will also be a bit less likely to freeze and thaw severely when the spring temperatures swing up and down.
I am concerned about overwintering the plants in containers outside. The roots will not be as well insulated as they would be in the ground so they may not surive. You may want to bring them into a sheltered but cool storage area in late fall and bring them back outside again in early spring after the worst of winter is past. During storage you would need to keep the soil slightly damp, not wet and never bone dry. If it is possible, I would recommend planting them in the ground rather than keeping them in the containers.
As far as soil mix, use a good quality soilless potting mix formulated especially for container plants. Your nursery staff should be able to recommend a brand that is carried locally. Good luck with your boxwoods!
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