I completely agree with Andrea, Nossie and Novaguin. 5 gal tank is faaaaaaaaaaaaaar tooooooooooooo smalllllllllllll.
Just to give you an idea about how quick they grow: we bought our first goldfish ( Comets )in March this year, they were about 7 cm including tail fin. Now they are about 18 cm including tail fin. So more than doubble their size in a bit more than 1/2 year.
Apart from this it is muuuuuch easier to maintain a big tank than a small one.
Another advice: before you get any fish, please make yourself familiar with the Nitrogen Cycle and how to set up a tank anyway. But you may already know.
If not please don't hesitate to ask, as we are on here to help.