I agree. I don't think the shape has much to do with anything except catchlights.
In considering a softbox, I'd first decide if it's use was for head and shoulders to 3/4 or for full length.
If for the former I'd stick with about 24x36. If the latter then get the large/extra large size.
Remember the closer the light is, the softer it is. In my opinion the very large ones give a very muchy light when very close.