Keep in mind that I am not a "bodyman" who does alot of steel car blocking, but I do tooling and modelmaking for fiberglass.
I have a 4' long by 6" wide sanding board that I made. Semi flexible. I use sandpaper from a belt sander. (cloth backed resin bond sandpaper lasts forever when using it by hand) I glue the paper on with feathering disc adhesive so it doesn't wrinkle..
I make board the conform to the curve by pulling the handles together. It does require some "feel" to work it correctly.
I have found it to be indispensable when I am doing tooling for roofs, and tonneau covers and large, one off custom panels. I would not have been able to make the streamliner body in my photo album as straight as I did, (faired to .040" in 4',before priming it with 40 mil of sanding primer), without it. It is a fairly exertive activity to push that board all day.
I do use it for the shaping all the way down to 80 grit, then I switch to my shorter boards.
I personally would be lost without my 17" flexible, my 17"rigid or any of the other shaping boards that I have or have made.
The older pre war stuff with fairly severe compound curves are really hard to get wave free without the flexible ones..(for me anyway)
I did some consulting a few years back to a boat builder in the carribean islands, (yes ,someone paid me to help them), where I saw guys using "gang boards" 12' long and 6 inches wide. They were semi flexible.4 to 6 men would work together sanding fair these 65' catamaran boat hulls. (you gots to have rhythm, mang) There is paper available for those boards. Boat shops carry it.
The "wave sanding theory" I learned says: In fairing large objects you want a board that is as long as 2 times the distance between the longest wave you are trying to sand out.
It makes sense to me. But that's just me.
I would say that long boards have their place.