A white reflective umbrella. Use this size for a 1 - 2 person 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.