Normally, a bedroom needs to have a closet and a window. However, some rooms with a closet next to it but not in it are being called bedrooms. Also a "bonus" room may not be considered legal depending on the 3R report or if you know ahead of time it was built without a permit as Kevin mentions. You might be thinking of a "sunroom" which is usually located off a bedroom in the rear of the house. The sunroom with a window but not a closet should not be called a bedroom but often is by some realtors. Size does not matter except in the eyes of the beholder.
Happy New Year!