Determine how big and small the window can be. Consult with your builder or architect to see how large a window is possible to create a maximum. Find out what the regulations are on minimum window sizes. Building codes usually require bedroom windows to be large enough to provide escape if necessary as well as sufficient ventilation. Find out what your local requirements are.
Consider what direction the window will face. Windows that face different directions will get varying amounts of sun during the day. A window that faces west will get a lot of afternoon sun and will heat up the room considerably. In hot climates, this may be a detraction but in cooler climates, a plus. North windows get diluted sunlight, south windows get more direct sun and east facing windows get morning sun.
Factor in the proximity to your neighbors. If you have a lot of neighbors within close proximity, you may want a smaller window that gives you more privacy. If you are in an open, private area, large windows may be ideal.