Under most circumstances, you'll find that the standard size of a king-sized headboard is 78-inches wide. It isn't uncommon to find most headboards, including mission style, to fall up to three inches wider than the bed's frame and mattress. The average size of a king-sized mattress is 76 inches wide, so you can see the difference. However, this isn't a bad thing. You'll be able to see more of the headboard's details thanks to the width and this slight reveal.