A single portrait light must be at least three feet wide and tall in order to have the light wrap around the skin for a flattering look as well as to supply visible modeling to the features of the face. Obversely, a source of light that is too wide will erase any benefit from shadows and modeling. The soft box is an ideal source of portrait light because the direction of light rays emanate from all points on the surface of the diffuser equally. Skin lit by this light appears smooth, yet filled with detail.
In order to balance the single light with the correct amount of fill light, the walls should be painted a warm white. If there is a built in gobo or dark shade around the light, I recommend that it be removed so that the diffused light can reach the walls and ceiling with enough power. The ratio of main light to fill light can be adjusted by pulling the light back from the subject for more fill or moving it in closer for more modeling and a higher ratio. A soft ratio of two to one is beneficial for baby photography, high key portraits and flattering portraits of older people. A more normal ratio of three to one serves the average person well with good modeling and open shadows. People with handsome features often can take advantage of a four to one ration for a strongly modeled effect that makes a powerful photographic statement.
One tool that compliments the single light setup is the Hasselblad Softar I portrait filter. It’s dozen or so tiny surface lenses spread the highlights without affecting adversely the contrast. This smoothes the skin, emphasizes the highlights in the hair and gives life to the eyes. When all the factors come together, you will discover a new beauty in your portraiture.