First know which wood is which on the board


Then measure that wood's width, you don't need an exact number unless you're going for 100%, in this case it's around 7.25 inches


Then measure on the board and divide, here it's around 9 inches



now you divide 9 by 7.25, which is around 1.25, so the board is x1.25 bigger than the wood.
so if the wood is 10 inches long on the chalkboard, it needs to be 8 inches. (10 / 1.25 = 8)