If it is in the lower level it may not be considered living space by an appraiser. Although useful to a buyer, not usefull in rasing the value in the banks eyes other than a finsihed basement. Average bedroom size depends on the type of home and if in a starter home, middle class home or luxury home. Sizes of 12x12 are considered good in most moderate sized homes. Make sure their are doors, heat, window and a means of egress to the exterior to meet your local code.