What frustrates me is that so many of these commercial panels are outside the really handy size. Two 4x8 panels work pretty well. To be clear, what I am saying here relates to photographing people specifically. I find I use different strategies for tabletop, etc.
Tall: V-flats go down to floor level and are above anyone's head. The angle of the light is just going to work. 7 feet tall is probably enough, but 6 feet can be a bit short. I often feather the key and count on the panel for fill. Because the light is high, I need the panel to throw back light on a high level. Over 8 will challenge ceilings at many locations. Ceiling fans and lamps are an issue and some interiors don't really have a full 8 feet. So 84-96" is the best range and 90" might be the best for location use. In the studio, v-flats built to your space are better than constructed panels.
Wide: 4' for each panel has worked quite well for me. It is rare two 4' panels together (one v-flat) isn't enough (at that point, another one comes out). I have run into situations where wider than 4 feet has created problems getting everything to fit and has created inconvenience. I have not run into space problems with v-flats. I think 3 feet is the bottom end of useful. Add much more than 6" to the 4 foot length and there are too many situations where it starts to get in the way.
I would say two hinged panels that were 48x90" would be the most practical and versatile size for location use. 36x84 would be ok and as small as I would want to go, but 40x84 would be better (I know an extra 8", 4" per side, doesn't sound like much, but still...).
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