I am thinking about adding a favia into my tank, but I was wondering if they grow into a ball by themself or should I take some action to help them out.

I have had one in my tank for 6 years...it has not grown into a ball.

favia is a big genus- certain species may be more prone to balling, but it think it also strongly depends on the conditions of the tank/water.

It also depends on where you put it. Generally it's tendancy os going to be to spread out so if you put it on a flat surface it will encrust more. YOu put it on a small rock or unveven it will tend to "ball" more as it grows.