What size mini split does a 200 sq ft bedroom in Arlington, VA need?

Notes

• The "20 BTU per square foot" rule is a Midwest number. Arlington runs 1,868 cooling degree days, which works out to 23.6 BTU/ft² — so for this bedroom the rule is 18% short. A single national figure has to be wrong somewhere, and this is where.
• Bedroom: the room where oversizing hurts most — quiet, low load, long runtimes wanted.
• 6,000 BTU/hr is 27% above the load, and the gap between head sizes is wide — this is already the smallest head. In a single room that matters more than it would on a central system: there are no other rooms to absorb the excess capacity. If the room is well insulated and shaded, the smaller head is usually the better buy, and modern inverter compressors modulate down far better than they modulate up.
• Arlington is moderately humid — summer nights stay within 17°F of the day's high, the signature of humid air. A meaningful share of this head's work is removing moisture, and it only removes moisture while it is running. This is the climate where an oversized head produces a room that reads cold and feels clammy.
• Do not forget the winter side. Arlington designs to 16°F. A cold-climate rated head will hold most of its capacity there, but a standard one will not — ask for the capacity at 16°F in writing.
• Where these numbers come from. Washington National Airport, 7 km from downtown Arlington, daily records 2015–2024. Design temperatures are calibrated against published ASHRAE values at eight airports and land within about 2°F of them.