They grow very large in ideal conditions. They are probably easier than O's for a 55 thoughAlthough Franks photo is a wild fish, goldfish kept in good conditions will (and should ) grow quite large.
I believe the fact most don't grow, is because some aquarists don't provide conditions needed. Even the fancy type should be given large enough quarters to reach potential.
The one below started out at 0.5" a little over 2 years before this pic was taken.