It's actually quite possible they will never get larger than they already are. Micro stars are quite common hitchikers and don't get much bigger than 1" at most. If indeed juvenille stars and not micro's, it will really depend on the available food sources to how quickly they will grow. If after a few months there is no noticable change in their dimension, they will most likely be micro stars.
the little white ones are probably micro's wich grow and split quite fast but never get very big most of the bigger stars grow quite slow as far as i can tell, and brittle type stars seem to grow fairly fast.
Depends on how often and much you feed them. If you let them fend for themselves, it may take a long time. Most aquariums have plenty for them, but if you feel the need, a small pieces of shrimp once or twice a week will speed their growth along. Good luck!