Why a post about a bedroom air conditioner...well, that's where you will be most miserable without one. I can post other applications later (or not)...
Let me make this ridiculously easy:
Under 175 square feet, a 5,000 BTU(BTU is British Thermal Unit) air conditioner will be adequate. Under 250 square feet, a 6,000 will be adequate.
Do not oversize an air conditioner. It will not be able to dehumidify and will actually freeze up.
Sunny or Shady: Add or deduct 10% accordingly.
Tall Ceilings/top floors(brownstones, etc): Add 2,000 BTUs.