I believe that Uppercase Letters 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 any child when they see a Lowercase Letter written larger than an Uppercase Letter.
Instead, I find it fun to refer to the Uppercase as the “Daddy” Letter and the Lowercase Letter as its “Baby.” This idea helps the young child understand that they are like a family and are the same letter just a “Baby” and a “Daddy” version of the letter.
The technical terms can later be taught when the child is older, like around 3-4 years of age. But by explaining the idea of each Uppercase and Lowercase pair being part of a family, young child can understand that the Upper and Lowercase version of each Letter is a pair and when the child moves to Letter Sounds they will not be confused when the Upper and Lowercase of each Letter produces the same sound.