In a large pond or lake goldfish will grow very large compared with the little ones you buy. My Hamlyn guide says up to 3 kg (about 6 lb 9 oz) in the wild in Europe.
The common goldfish is Carassius auratus, and is a species of carp. There are quite a few other carp-family species which are kept as ornamental fish, and many of these have golden (leucistic) forms. I know of common carp Cyprinus carpio, tench Tinca tinca and orfe Leuciscus idus, and perhaps Crucian carp Carassius carassius and rudd Scardinius erythrophthalmus. Koi carp are the same species as common, mirror and leather carp, and they too can be golden, as well of course as many other colours.