The four parameters of the Create Fishnet tool work as two pairs. You can use cell size (width and height) and extent of fishnet to determine the number of rows and columns. In this case, you enter zero for rows and columns. Similarly, you can use number of rows and columns, and extent to determine the cell size. In this case, you enter zero for width and height. However, you cannot enter zero for one parameter from the first pair and zero for another from the second pair.
Consult How Create Fishnet works for details with graphic examples.