This is a modification of the Farmhouse bed with a panel headboard and footboard rather than individual boards. It currently fits around the mattress/boxspring/metal frame. When we buy a new mattress, we will then add the joists, etc.
The headboard and footboard are both 72" wide. We had the guys at blue do a rip cut at 72" which ensures that both pieces are exactly the same measurement (for square.) Then, they turned the plywood a quarter turn and cut at 15" for the footboard. This left a 33" high headboard. It is 3 inches larger than the original plan, but it doesn't require a second cut and you don't see it anyway. The trim is just a 1x3 with a 1" overhang. (I might make it just a 3/4" if doing it again.)
The side boards are 1x12 cut at 80 1/4", but you may want to add a little if your comforter is really thick or if your mattress is much longer than ours at 79 1/2". We used non-mortised (no cutting into the wood) bed rail hardware from Woodcraft, so we could easily take it apart. These were placed 1 1/8" in from the inside edge of the 4x4 post. They were placed on the very edge of the side rail so it would butt up against the post. The 1 1/8" gives us plenty of room to shove the bedding down inside the side rail.
We glued and nailed 1x2s from the back for the trim on each piece.