Check out some of these Thailand Fishing resort sites. You'll see Hypsibarbus Wetmorei the yellow fin tinfoil barb hit 15"-20" easy //
https://www.fishsiam.com/species/barb-golden-belly-2/
This one here spans her beyond shoulder to shoulder probably just at 20"
http://www.palmtreelagoon.co.uk/golden_tinfoil_barb_gallery.html
In the 6th pic that barb is over 20"
http://huahinfishing.com/the-fish/
these here i guess are pretty average
golden belly tinfoil looks bigger than schwandenfeld barb in every one + most places that keep giant siamese carp will have both barbs there most likely also huge.
20" for tank raised barbs will be a challenge but they're out there. would be sick to keep the puntioplites proctozysron.
i am quite sure none of the barbs you mentioned are over 20".. the pic was taken with hand outstretch to put the fish closer to the camera thus making it looks bigger...