Hiya aquakitty, welcome to MFK,
I have not kept many goldfish, at least for more than a couple weeks but there are several GF keepers on the site. What type do you keep? the comet style gold fish has been recorded as large as 24" but you have a very long time before worrying about that, a 50G tank would be good for several years before a pair outgrows it. Not having a heater is a good thing if you have one of the hardier types but some othe fancier types require one, common gold fish can survive (but not thrive in) waters from just above freezing to around 100 degrees F. I do suggest either adding a UGF filter or another hang on filter rated for at least 30g when they reach around 6-8" or so, they are messy fish and need good filtration.
One good thing about goldfish is that treated right they can still be around to entertain your future grandchildren.
