Answer: Boxwood is slow growing, just a few inches a year, so it would only be suitable if you are a patient gardener. The varieties Green Mountain and Green Velvet (varieties suggested by the Chicago Botanic Garden) are listed as growing to between three and five feet and would be naturally rounded -- but can be pruned if needed. It does require protection from wind to grow well and minimize winter browning.
There are a number of plants you could consider. Certain varieties of Chamaecyparis (naturally rounded) along with mugo pine (can be shaped by clipping the candles early each spring), or yew and hemlock (these would require regular pruning) could all be possible choices. Of these, yew would probably be the easiest and fastest one to grow and the most amenable to pruning to maintain a specific sculptural shape.
I hope this helps you with your decision.
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