Here is what a simple 28 x 28 inch softbox with a fabric egg crate grid looks like, with the subject standing about five feet in front of a very pale, gray-painted living room wall. The softbox was about two-arms-outstretched or a "wingspan" away from her. This was in a low-ceilinged living room, with just one Speedo Brown Line flash at 50 Watt-seconds (full-power on say a Nikon SB 800 flash) firing inside of a low-cost Made in China, no-name softbox with is 6-row x 6-column fabric egg crate grid. Shot last week, with a 50mm lens at f/10 on an old crop-frame Nikon from 2005
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I left the forehead UN-smoothed, and the close arm as well, so you can see that the light is mostly soft, but it will reveal some detail as well; if this had been shot with a beauty dish, the skin detail and the shadows would have been harder. I took the Lightroom brush and applied some minus sharpness to the wall areas around her, but most of the rest of the frame is un-retouched, so you can get an idea of how a small, 28" x 28" grid-equipped softbox looks at 67 inches or so from a person. The 3 x 4 foot softbox would have been a bit softer, but also likely would have "lit up" the wall behind her more.
Personally, I kind of prefer the more "shadowy" look of the 28" x 28" boxes to the 36 x 48 inch boxes, especially if I am going to convert to B&W, as I did here. I think for lighting in black and white, a crisper, more-shadowed look reveals shapes better,and looks better too.