A large number of 4x4 word squares have been devised containing 16 different letters of the alphabet, a 5x5 square with 22 letters has also been constructed, and a 6x6 square with all but Q, X, and Z (see Word Ways, February 1980). If a square containing every letter of the alphabet (a pangrammatic square) is ever found, it will almost certainly be at the 6x6 level.
"A Pangrammatic Six-Square,"
Word Ways: Vol. 26
, Article 8.
Available at: http://digitalcommons.butler.edu/wordways/vol26/iss2/8