Ok, I've been browsing these threads & read posts saying '1 fish per 10 gallon', but not how big the fish have to be . Like do you mean a 2 inch fish per 10 gallon or what ?

Fancy goldfish need a minium of 10 plus gallons per fish cause of their adult size. Yes it may seem like much for a 2 inch fish but these fish dont stay 2 inch.
Now if you are talking tropical it varies. You can keep a ton of guppies in 10g. But something like an oscar needs a 55 gallon tank to himself or more.

So its just for them to grow, or it something like a health hazard?

Both, Have you read the Ten Steps to a Healthy goldfish??

This will explain allot of it, goldfish are big and they are big wasters meaning that once they eat it comes out the other end in a very short time, they need allot of room and allot of filtration they are part of the carp family, so if you seen a carp or a koi this will show you in your head just how big these guys get.....I have one that i bought 6 years ago and he was only 2" now hes 8" long and still getting big on me hes now a 8 year old...... So the saying goes the bigger the tank the better off the fish will be

Very helpfull, thanks. I'm kinda new here, so I don't know all the ropes.