Your kid will take a lot of time when it comes to understanding the order of certain things and elements. But as he reaches the age of 6, he becomes acquainted enough to identify objects based on it's size. Now if you will ask him to differentiate between a tennis ball and a football he will identify them based on it's size.
What you need to know:
There are chances of your kid understanding the extremes like biggest and smallest, it is important for you, as parent to help them identify that there are objects that fall in between. Play block games with your kid that require to arrange objects in order of their size to help him improve his skills.