It isn't all about size.
Size (or relative size) is just one consideration. A large softbox placed close can produce soft frontal lighting, or in a rimlight situation can produce high local contrast even when close. In other words, a large softbox can produce either low overall contrast or high local contrast. A smaller softbox, or the same softbox from further away, can't produce such soft lighting.
But that doesn't mean that a larger softbox is always better. Sometimes you don't want the light to be too soft. OK, you can move the softbox further back to make the light smaller and therefore harder, but by moving it further back the light you also change the rate at which the light intensity falls off.
Another consideration is shape. Different shapes produce differently-shaped catchlights.
Another consideration is feathering, if the softbox is deeply recessed to allow feathering. only square or rectangular softboxes can be used to feather light accurately or to create graduated lighting, say on the background.
The there's ceiling height. There's little point in having a big softbox if the ceiling is too low for it to be positioned high.
:thumbsdown:
