How big should a fish tank be for two gold fishes?

It depends on how many goldfish and their size. Just call or stop into a pet shop within your area, and they will be able to advise you appropriately.

Answer; generally as a rule you should allow 1 gallon per inch of fish The above answers are okay, however I have found that goldfish tend to thrive when they are given at least a 20 gallon aquarium to live in and an additional 10 gallons for each additional fish.