This Hensel Ultra Silver 105 is a 42", hot silver umbrella; its size is perfect for a 2-person head shot, or 3/4 length single portrait.
Umbrellas soften, broaden, and diminish the light output of any tungsten or flash light source. Umbrellas with a white interior will soften and weaken light more than an umbrella with a silver lining. A white umbrella without a black backing may also be used as a makeshift softbox, although the use of the light will not be as efficient as with a lightbox.
Choosing umbrella size is determined by the size of the subject, and the strength of the light supply being used. Choosing an umbrella surface is as subjective as choosing a paint and brush.