Comets fair enough a pond would be the best bet, but for the average fancy goldfish wont get half that size. Some might, course. The trouble you'll face trying to change something like this is that it would be too damaging to the industry to enforce. Imagine how much would be lost on sales if there was a minimum tank requirement.
I'm all for stopping fairs giving fish as prizes, and goldfish bowls etc. I'm just not sure how do-able it would be to enforce minimum sizes of around 4 foot! Maybe if shops (like many do) would refuse sales for unfiltered tanks that might be a more practical start.
Tom