When you place a cake on a decorative board, it is usually 2 inches bigger in diameter than the cake size--is the rule the same for wedding cake separator plates? You don't want the plates to show, so do you just get the exact size of the cake (the borders go on top of the next layer, right?).

If you are doing cakes that are tiered with columns, you can have the separator plate either way--showing or not. That would depend on whether you had to pipe on a bottom border or embellishments around the base of each cake.

If it's stacked, I wouldn't think you'd want the board to show, so they'd be the same size as each cake, or just SLIGHTLY smaller.

HTH,
--Knox--