"Growing to the size of the tank" is what is known as stunting. A cruel and painful process which causes internal damage to the fish, leading to premature death. I've seen the strangest deformed looking fish that have been stunted. It is not natural and it is not humane. I understand your confusion. Trust me when I say this, take EVERYTHING the people at any pet shop say with a grain of salt. Ask all of your questions on a reputable forum such as this one, and do your own research online as well. Never stock your tank with fish recommend there at the store. Pick out what you like and then find out on your own if they are suitable for your tank.
So now you have a 7 gallon and a 16 gallon correct? Have you gotten rid of your other fish? That's what I would recommend. It's definitely time to make sure you know about cycling. Cycle your tank before adding fish, and all your fish won't die in the first couple months which is what happens to virtually everyone who doesn't cycle first (take it from me, I learned the hard way!). To get an overview of what cycling and the nitrogen cycle are, read here: The Aquarium Nitrogen Cycle
Here's a good guide to doing a fishless cycle: The (almost) Complete Guide and FAQ to Fishless Cycling
If you haven't done this and just put fish into a tank it's going to through the cycle with fish in..yikes..then you need this: I just learned about cycling but I already have fish. What now?!
If you haven't already realized after reading these, one of the most important things you need right now is a freshwater master test kit. Also a bottle of pure ammonia.
I know it all seems a bit complicated and overwhelming right now, but once you get the hang of it keeping fish the right way is a really fun and exciting hobby