This is an Impact 30" White Reflective Umbrella. Umbrellas soften, broaden, and diminish the light output of any flash light source. Umbrellas with a white interior will soften and weaken light more than an umbrella with a silver lining.
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.