I'm finishing off a bonus room upstairs, which is roughly 370 sq ft (26 ft x14.5 ft). I'm planning on installing a mini-split system instead of tying into my existing heat/air system (don't think the furnace/AC is big enough). I'm not sure on what to go with in size, 12KBTU, 15KBTU or 18KBTU, any suggestions? I live in Oklahoma so the winter's are typically mild but the summers are mostly scorchers. I know heat rises so I'm not too worried about keeping warm, I'm more concerned with keeping it cool.
I have another question on where to mount it. The ceilings are built on a slope to follow the roof line so I have vertical walls that are about 4-5' high with the ceiling sloping up to 7' high and then it is straight across. What height and where in the room? The stairs will be open to the downstairs and they are on one end of the room, not in the middle.