It's probably not what you want to hear but the general rule of thumb for goldfish is that you need 20 gallon for the first fish and 10 additional gallons for each additional fish. So that would mean 40 to 50 gallons for 3 or 4 goldfish. Goldfish can grow to over a foot long and live for 25+ years if properly cared for. They are extremely messy and create a hugh amount of waste thus needing good filtration and a good maintanence program. They also need lots of swimming room. If you are limited on space, I would suggest a smaller tank with smaller community fish.