Well they grow less and slower kept inside in a tank so it could take a while. They could live in a 29 gallon or larger, depending on what you want to spend, ideally a 55 down the road if they get over 3-4 inches can comfortably house 4-6 with regular water changes.
Ponds require a whole bunch of new knowledge, and care. If you freeze over your fish need to come in in the winter or your pond need to be deeper than 3 feet. You need to run filters, and maintain a whole new ecosystem, so make sure that's something you want to do. I personally prefer the fish tanks. They would probably multiple in a pond too, mine did their first few seasons.
Mine grow like weeds outside but more slowly inside so it could take years for them to get bigger. You certainly could set up a cow tank or something similar to put them out in summer with a small pump and filter and manage it like a fish tank with water changes.
Just depends on what you are interested in and what you like doing.
