You have three choices in addressing your large postcards. Large postcards are 5x7, 5.5x8.5, 6x9, and 6x11. The shaded areas are reserved for addressing purposes, the white areas can be used as desired.  The examples below are based on 8.5x5.5 postcards, but the measurements indicated for addressing remain the same if you are using one of the different size postcards listed above.