Most places give the size of adults as 17-24 inches. There is a reasonable amount of variation due to genetics, just as people can be perfectly healthy at a range of heights, but 14 or 15 inches probably below this range.
Both my bf's adults are 17 inches, which is on the small end, but he rescued them both as adults, so I'm pretty sure they weren't fed as much as needed when they were young. The male was 18 months old when we got him, so we assumed he'd be fully grown, but 6 months later he is still growing a little bit lengthwise, and putting on weight.
This isn't to say you shouldn't adopt a small beardie, you might be able to make a big difference to its health, and even see it growing once on a proper diet, but chances are the ones you've seen haven't been properly looked after by their current owners (maybe due to the stupid 'they only need 4 crickets a day' or whatever rule pet store employees tell people).
