Within the International Residential Code (IRC), a room must have certain features before it can be called a usable bedroom. Most of these features involve both safety and livability.
While in college my bedroom for the better part of a semester was literally one of my house-mates oversized closets. While I paid rent for this bedroom it was a little too small by most of today’s building code standards. There are many interpretations of building codes in the United States today. Most codes however, describe usable, habitable bedroom space as having 7.5 feet of head space with a minimum room dimension of seven by ten square feet. Bedrooms can be invented in a variety of spaces but they might not have enough headroom needed to qualify as a true bedroom.
Habitable bedrooms must have at least 2 exits, at least one of which must be a door. The second exit can be a window that also must be a minimum dimension. The current IRC sets the minimum opening area at 5'7" with a minimum opening height of 24 inches and width of 20 inches. You will want to check your local building code for specifics relating to your situation.
Building codes take all sizes and shapes depending on where you live. Because of this there is no silver bullet when it comes to making that unused space usable square footage. Keep in mind that you can do almost anything to your current home without anyone but yourself knowing. However, if you are going to sell your home at some point in the future, a change will almost always get picked up by a good home inspector.