What size is one half of an illustration board?

One half of a standard illustration board, which typically measures 32 x 40 inches, would be 16 x 40 inches. If the board is cut in a way that divides it into two equal parts along the shorter side, each piece would be 16 x 20 inches. The size may vary if the board is a different standard dimension, but this is the common measurement for illustration boards.