They really do grow fast!My pet shop Syrian went from 2-3 inches when I first bought her to around 4 inches within the first two months
That same hamster is less than six months old and already 5 inches long when I last measured her,so don't worry that a bigger wheel somehow won't work or that she won't grow into it.If anything you save yourself from having to upgrade a wheel unless she manages to be a six inch long hamster,which is still very possible!