It is possible to buy the smallest window for bedroom installation but let me first point out that there is a fire code on the size of bedroom windows. There has to be an easy exit in case of fire. There is a company that claims to make what is probably one of the smallest egress windows.
It can do this because of a tilt/turn so the window opens into the home like a swing door which makes it easier to get out of in a hurry. It is 380mm high and 380mm wide with an area of 0.35.2. This is probably one of the smallest window for bedroom usage.
The International Residential Code which is nation wide states the width should be a minimum of twenty inches, the height a minimum of twenty four inches, the net clearing a minimum of 5.7 square feet or 5.0 square feet for ground floor, the sill height above the floor has a maximum height of forty four inches so watch it when installing the smallest window for bedrooms because it may not pass inspection.
There is also a code for a minimum window size relative to room size that is nation wide. If you have a room, even a small room which you think you might use as a bedroom some time make sure it has at least one window with the legal egress requirements for a fire escape.
If you are buying a home make sure that the smallest window for bedroom installation meets the legal egress requirements. Some older homes were built before the egress code went into effect and sometimes the newer homes also lack them.