Answer: Boxwoods grow less than a foot a year but they do eventually reach a fairly large size. Green Mountain is potentially five feet tall and three feet across. The container should be in proportion to the top growth and the soil mix should be a fairly close match to the soil it came with originally so they drain at the same rate. Generally speaking, the larger the container the better. A small container will dry out very quickly and the small soil volume allows it to heat up in summer and be overly cold in winter as well as go through damaging freeze/thaw cycles.
I should mention that this plant would not be recommended for container growing in a cold winter climate such as yours. The roots are not well insulated as they would be in the ground, and the exposure to extreme cold as well as oscillating temperatures stress it terribly.
This boxwood variety is considered winter hardy into zone 4 so I would strongly suggest you plant it in the ground now if you can (You are located in zone 5A or the colder part of zone 5, it may be as cold as zone 4 depending on your microclimate. Select a site with protection from winter wind.)
If you can't plant it in the ground, in winter you will have to try to protect it by bringing it into a sheltered but cool place such as an unheated garage or cool greenhouse with temperatures above freezing but below 40 - 45 degrees. Bring it in in late fall after freezing weather has stopped its growth and take it back out in early spring so it can wake up naturally with the season.
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