RE: What if your pet grows and grows and grows ...
As far as aquarium fish is concerned, there is one which definitely needs a lot of planning for extra space as it grows. I am talking about the Arowana.
I had reared a few before. They started as six-inch tiddlers. Then they started growing at the rate of at least an inch a month. In less than one year, I had Arowanas nearly 2 feet long. Which means that they need tanks not less than 5 feet long. On top of that, Arowanas don't live together. Put two into one tank and one will be dead before the day is over.