Goldfish grow big. Depending on genetics they can grow to anywhere between 8 inches and 18. The average around a foot.
You have to think ahead when buying a tank, do you want one for the rest of their lives or keep upgrading as they grow.
Getting a tank for goldies isnt cheap but if you get the right filter then you can economise on the tank size. Filtration is dependant on the bio load the filter can hold. The more biological media the better. A juwel tank is great for the media but it does mean you will need another small filter to keep the water circulated.
For your two fish to see them through to adulthood you need around 50 gallons.
Of course you need lots of money and room to do that and not everyone can afford to.
You budget for the biggest you can afford and make sure filtration is good and with regular water changes and maintenance then your little fishes will be good.
You can get away for a long while with them being in a 20 to 30 gallon, but make sure you have test kits, cycle the tank first and do regular maintenance.
Theres always ebay or the classifieds where your money will go further, you dont have to buy new.