yeah some stores do have labels half of them are rubbish though, if yours don't then there's 2 good things you can do to help you out.
1 - invest in a really good fish book, with sizes, fish requirements etc in, you don't need to memorise it, just take it to the lfs with you, if you like something then look it up.
2 - ask the lfs to reserve it for you, then go home and do your research, if it's not suitable then call them to say you've changed your mind.
there's a few way's a fish can die from being in a tank too small-
it will stop growing on the outside when it reaches about the right size for the tank..... but it's internal organs won't and they'll all keep growing and get squashed up inside it basically until something bursts and it'll die (i know, gruesome isn't it!)
it will get overly agressive or territorial and end up fighting and dying from injuries or stress
it will simply continue growing to the point it won't be able to turn around in the tank and get food, so it'll die form starvation and lack of exercise
but by far the most likely is it will become stressed by being cramped, this will make it susceptible to diseases and it'll pick something up that will kill it, and maybe half the other occupants of the tank too.
Unless your gonna get a seriously bigger tank and soon then you need to re-home these fish.