The Goldfish and Aquarium Board (linked from the Goldfish Society of America) notes that:
He may look like a tiny thing but goldfish live long lives and grow continuously.
This BBC article tells of a woman who had a 2lb goldfish.
(From the same article):
But Dick Mills, secretary of the Federation of British Aquatic Societies (FBAS), said Goldie's size was "not that unusual".
"I would think there are probably a few bigger goldfish that people don't think of as record holders, perhaps in ornamental lakes.
"Goldfish are very long lived animals and consequently if they're in the right environment they will get large."
The Federation of British Aquatic Societies single-tail goldfish care sheet notes that they can grow up to 350mm (almost 14 inches) but rarely do as most are kept in aquariums. The Pond Care sheet also notes that "Pond Goldfish will grow to around 200mm/8" depending on the size of pond as they tend to adapt their size to the environment."
The goldfish websites did note, however, that in order to be happy, healthy fish they do need at least a 10 gallon tank. As the link you sourced noted, small goldfish bowls are unhealthy and do not allow proper space or gas exchange.
If you want a goldfish the size of a shark, though, I think you are out of luck.