Depends on the size of the fish as it grows. For an adult common that say maybe gets to 8-10 inches then i would personally say 50 gallons to allow for any more growth and swimming space.
But if you can afford to upgrade as your fish grows then for a fish thats between 1-3 inches then 10-15 gallons, 3-5 inches 15 - 25 gallons and 5- 8 inches then 25-45 gallons seems reasonable to me, but as goldies grow so fast then this may become very expensive.
Of course thats my opinion and also the fish needs the best filtration. Personally i think once a goldie reaches 5 inches it should be in a pond where it can easily get to over a foot if genetics are good.
A lot of people keep goldies in smaller tanks and this results in the fish becoming stunted which to me is unfair.