First of all, it's a good thing that you ask this. Generally, I would say that experience is rather important for deciding which fish will fit in which tank. Over time you'll learn this out of your own experience.
Secondly do I have to warn you. Never, ever rely on the advise of your LFS. They are often incorrect and will almost always tell you that it's ok to buy. So always do research upfront. This brings us to the problem of differences in information on the internet. This can indeed be pretty hard. However, if you read on multiple sources you can find the basic ideas of this fish. On some points do most of the sources agree.
Try to find certain sites that are reliable. There are a lot of them and most will pop at the top 5 of a search on google.
Your fish will indeed become about 12 to 15 inches long. The exact length might differ a bit since not every fish of the same species will grow to the same size. Just like that people differ in length, so do fish species. So take it as a guideline. This means that, in my opinion. Your fish will become too big for your tank.
As you mentioned there are some people that think that fish will grow to the size of the tank. This might be partially true. They will reach a slightly smaller size but will still grow substantially.
f a fish is too small, it is mainly a sign of improper care. So, the wrong or not enough food, bad filtration, wrong water parameters etc. This also implies that if you ever see a fish that is too small, you can bet on it that it isn't healthy and will die very young. They will get ill much easier and are much often bullied by other fish.
So always make sure you can be sure that you can take care for it properly before you buy an animal.
I think it would be best to sell your fish or buy a bigger tank. This will reduce the stress levels for these fish. Furthermore, will decoration help. It might sound very strange at first but for most fish. The more hiding places the more often you will see them.
If you want to buy fish again and you are not entirely sure if you can keep them in your tank, you can always ask this on the forum.