I like the "rectangle the bottom" approach. Keyboard shortcut R gets the Rectangle tool. Start to Rectangle the bottom of the board. Type 14',1.5 (if inches are your default, you don't need an inch symbol). This assumes you want a fourteen-foot, 2-by-something. Once the bottom is modeled, Bucket with your chosen material or color, then PushPull up (to 5.5 if you are modeling a 2x6). If you bucket first, all six faces of the board will be finished.