they will grow as large as they can (up to their maximum possible size) depending upon water cleanliness and quality, food quality and amount, and overall health. a stunted fish (one kept in very bad conditions or kept in a tank too small) will not grow as large as a fish kept properly, but at the cost of the fish's color, activity level, immune system, and lifespan. a stunted fish will be more prone to disease and early death, not to mention one effect of stunting is that the outside of the fish will cease to grow, while the organs will continue to grow. organ failure leads to early death.
please never buy a fish with the intention of trying to stunt it.