Answer: You might be able to keep it that small, however "Green Mountain" is described as "Dense upright form with bright green leaves that retain their color through winter. Natural cone-shaped form if left unsheared, excellent hedge. Evergreen. Full to partial sun. Moderate grower to 5 feet tall, 3 feet wide. Cutting grown." So it would not be the ideal candidate. You might want to use one of the cultivars that is naturally closer to the size and shape you need such as Buxus x 'Green Gem? Plant Patent No. 3736, or Green Ice Buxus x 'Green Ice' Plant Patent No. 10773 (COPF) both of these are naturally smaller and rounded and very slow growing.
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