A common goldfish (not koi, they are larger) can grow to be over 12 inches long not including tail fin, over 6 inches from stomach to beginning of dorsel fin & 3 inches thick at center. This is why it is considered so cruel to put them in small bowls or small tanks. the smallest tank suitable for healthy goldfish is 20 gal, however within 2 years (goldfish can live up to 15 years) they should be moved to a 55 gallon tank (at least) utilizing an under gravel filter with powerheads as this provides adequate biological filtration. Without the massive amounts of substrate (gravel) for benifical bacteria to grow on & the powerheads to pull the water (& fish's rotting waste) through it, the fish will be poisoned by the ammonia spike caused by their waste, their gills will lose the ability to process oxigen from the water, and they will suffocate and die. However as goldfish are among the hardiest spicies of aquarium fish and the first domesticated fish, given the proper habitat they are widely considered to be by far the simplest fish to keep, with the least amount of required maintinance to keep them healthy.