How should I size an air conditioner for a room in the attic?
Do not go by square footage alone. When calculating heat gain/loss, exterior surface (walls, ceiling and foundation) are the main determining factors. In your case you need to consider all the extra heat gain from the roof and also that cool air falls. In an attic you'll get all that extra heat gain plus all the cool air from the air conditioner is going to drop down effectively pushing the warmer air up. You can offset that effect by keeping your door closed when using the A/C. Assuming the same amount of exposed ceiling square footage as wall square footage I would triple the "recommended" size based on floor square footage.

This answer is just a guideline based on limited information. It is by no means an accurate calculation.