When comparing sizes of octaboxes or umbrellas. It is easy to determine that (given the same distance from modifier to subject) a larger unit will produce a softer light than the smaller unit. In other words, a 45 inch octabox will produce a softer light than a 32 inch or 24 inch octabox.
However, does this hold true for softboxes?
I realize that a 45 inch by 45 inch softbox will produce a softer light than a 24 x 24 inch softbox. However, in general use, would a 24 x 36 inch softbox produce a softer light than a 24 x 25 inch model. Or will it just produce the equivalent softness over a greater area?