Your little darling is learning words at a breathtaking speed. Each day you would notice that he is using a new word that has been added in his vocabulary list. He now has a better understanding of the concept of shape, size, color and spaces. With better understanding and bigger vocabulary, he would be using the right size words at the right time.
What you need to know
Children this age can clearly differentiate between big, small, tall, short, etc. and would be able to group things accordingly. He would use the right word to discriminate them too. All the story books that you have been reading to him and the picture books you have been showing to him have helped in this understanding.