I'm just back from a trip to France and I adored all of the boxwood plantings in window boxes and containers all over Paris.
Most Boxwood will do well over the winter if you remember to keep the soil moist.. It is hard to remember to water when it is cold and dreary outside and we are cozy and warm inside, but if you water, the Boxwood should be okay. I would recommend Buxus sinica 'Wintergreen' (3'x 4'), 'Winter Gem' ( 3'-4' x 4'x5'), 'Green Mountain' ( 3'x5'), or 'Green Mound' (3'x3'). These are quite cold hardy and as you can see there is a range of sizes. Select the size plant that you are looking for. A large container will dry out more slowly thus use as large a container that is horticulturally and aesthetically correct for the site. Also, a plastic container is lighter and dries out more slowly that porous pottery.