I believe that Uppercase Letter and Lowercase Letters should not be referred to as big and small letters. I feel that big and small refers to the size the letter is written which is confusing to a child when they see a Lowercase Letter written larger than an Uppercase Letter.
For small children who are not ready for the technical terms of Uppercase and Lowercase, I find it best to use terms they can relate to. I use the term Daddy Letters for Uppercase Letters and Baby Letters for Lowercase Letters. The technical terms can later be taught but this way the child is not concerned about the size font of the letters.