Put in the smallest possible size board all combination of 4 quantity of letters. Crossword must be connected. And can be only 4 letters words. Cannot be words with 2, 3, 5 or more letters
Example for:
ABC = 3 letters = 6 combinations in a 3x3 square
Here is a solution for ABCD = 4 letters = 24 combinations in a 12 x 12 square = 144
I am sure it is possible a solution in a smallest area rectangle