- This code is not very DRY you are repeating the same things three time. You could turn the letters arrays into an object: 

           var letters = {
               small: ['A', 'A', 'B', 'B', 'C', 'C', 'D', 'D', 'E', 'E'],
               medium: ['A', 'A', 'B', 'B', 'C', 'C', 'D', 'D', 'E', 'E','F', 'F', 'G', 'G', 'H', 'H', 'I', 'I', 'J', 'J'],
               large: ['A', 'A', 'B', 'B', 'C', 'C', 'D', 'D', 'E', 'E', 'F', 'F', 'G', 'G', 'H', 'H', 'I', 'I', 'J', 'J', 'K', 'K', 'L', 'L', 'M', 'M', 'N', 'N', 'O',               'O', 'P', 'P', 'Q', 'Q', 'R', 'R', 'S', 'S', 'T', 'T']
             }