The world record I believe is 22 pounds. A 25# fish has been caught but it was foul hooked and the angler released it fearing a foul hooked fish wouldnt be a record.
A 16" bass is about 2 pounds. In other words, food for a large full grown bass!
Hence the "at least" and "likely more" indicating that a largemouth should surpass 20". If kept in an inadequate tank, I could see one ending up stunted and maybe around 18", but in a good sized tank/pond, I wouldn't be surprised to see one near 30"