I needed something large to fit above the faux mantel so Matt and I went hunting in our garage to see what we had to make it happen. (TIP: Always pick up good boards when you find them cheap at Restore and yard sales. They are so nice to have on hand when a project idea comes up).
We had a butcher block cutting board in the garage that measured 28″ wide and 25″ tall. Perfect for the letter board. It needed a good sanding to clean it up, but it was a good solid base to start with.