The board in the Designer (virtual board) should be a little larger than the carving area. The actual board size should be 1" wider and 8" longer than the virtual board. Use "Center on board" and say NO to stay under rollers. Since the actual board is 4" longer on each end it will stay under the rollers. If you answer "Yes" to stay under rollers the machine will expect a board that is 7" longer than the virtual board. This is just one possible scenario. There are many other techniques but I think it's the least complicated until you get accustomed to the machine and software.
EDIT: In actuality, if the board is 8" longer than the virtual board it really doesn't matter how you answer the "Stay Under Rollers" question. Saying NO just means that if your board is a little short it will carve without asking to scale but won't stay under the rollers.