Goldfish are wonderful--I've never had a tank large enough to own any for myself--but I recently had to intervene when I realized my boyfriend's parents were keeping a fist-sized ryukin in an 8 gallon tank.
The general size rule seems to be 20 gallons bare minimum for one fancy goldfish, and 10 more gallons for each additional fish. It's important to get a tank that is long and shallow as opposed to wide and tall. I highly recommend "breeder" style aquariums--they are longer, wider, but shallower than other styles. This allows for maximum swimming space and surface area for gas exchange, which is very important. A 30 or 40 gallon breeder tank would be excellent choices. 30G breeder tanks can be a bit hard to find, but they're out there! I snagged one off of craigslist the other day for 50$ with lighting and a filter.
The GPH rating on the filter should be 10x that of the gallonage of the tank--so a 30 gallon tank should have 300 GPH filtration. Even with this, goldfish need weekly water changes. They are very dirty animals. Aeration is also very important--many cold water fish, such as goldfish, are adapted to live in high oxygen environments since cold water holds dissolved gases better than warm water. Airstones aren't really sufficient alone--consider powerful filtration and perhaps a powerhead as well.
Keep in mind that this is not a fish you want to do a fish-in cycle with. You'll drive yourself nuts trying to keep up with their waste output--take the time to do a fishless cycle first and you'll save you and your fish a lot of trouble.
As far as feeding, you don't want to use flakes or floating pellets since fancies are so prone to swim bladder problems due to poor diet. Go for high quality sinking pellets and you should really consider researching home-made gel foods, because there really is much to be desired in most pre-made goldfish food brands.