There is actually a little trick you can use to get you in the right size that few people know - if you take the length + the width of the room it will give you a pretty good idea on the size (in your case - 15+13=28).
In your case it's a little odd because the room is so small but the ceiling is so high... In that case I'd go with the 36 which is a pretty std. size"
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Mantle and millwork above fireplace
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