Since the problem with mismatched tubes is usually trying to put a smaller tube into a bigger tire, and the rubber isn't up to the stretch, and becomes too thin in places.

Soooooo, I took a brand new 700X35-43C tube and puffed it up to just attain round size for measuring. It is self supporting at this stage, and measures 22mm outside diameter.

Therefore, it will not be stressed, won't be pinched, and can't be smaller than the tire that will surround it. The wall of the tube would actually be more resistant to pinching, because it is thicker.

In my case, with this brand new tube - it will fit in any cavity from 22 to 43mm.