Hexagonal boards are measured flat to flat, therefore you will need a box 2" bigger to accommodate them.
If you are stacking the cake and therefore want a board the same size as the cake (no edge showing) the you will need a board 2" smaller than the cake (as hexagonal cakes and tins are measured point to point.)
If you want a hexagonal board with an edge then you will need at least a board the same size as the cake (This will leave roughly 1 inch either side, providing the cake is centred.)
