Goldfish need a heck of a space, more than you'd think!
I pulled this info from
http://www.tropicalfishcentre.co.uk/goldfish.h tm
which will give you a rough estimate.
Many numbers are quoted as to the minimum amount of space required for Goldfish, ranging from 8 gallons to 15 gallons per fish. A more useful rule of thumb may be to allow a gallon per �� of adult fish. This allows for the variation in size between the different varieties, and a 6� fish would equate to a requirement of 12 gallons � about the mid-point between the commonly quoted sizes.
Equally important is the shape of the tank. Tall, thin tanks are not suitable for goldfish, the ideal being a long, shallower tank. This has the dual benefit of providing a large surface area to maximise the amount of oxygen in the water and of allowing the Goldfish to swim the long lengths that they require in order to exercise properly. As a minimum a Goldfish should be able to swim 6 times its body length in the aquarium