Don't get me wrong, I love goldfish, but I know I haven't the space for a 55 gallon so I don't keep them. The OP should consider doing the same. It isn't rocket science to look at large full grown goldfish in ponds and correct sized tanks and see that they are exactly the same as the ones in petshop tanks. Ergo, it is pretty obvious how big they get and if you don't have room for it, don't get it.
I know I come across as overbearing about things, but I don't like to sugarcoat the situation to make it sound ok. Because it isn't OK. Seriously it doesn't take a vast amount of brainpower to figure something like that out.
But well, it's his fish and his own conscience, things will be what they are.