Our full size bathroom has a shower/bathtub that runs perpendicular to the bathroom length, and the tub is the standard 5' length. Thus, when you walk into the bathroom, you have the 24" deep vanity to the left of the door with a single sink, then a toilet, then the shower tub against the back wall. The dimensions of the room are only 5x8.
A 15x15 space can be chopped up for this, but you will end up with a bedroom that is L shaped, (I would use the leg of the L as a closet) and the bedroom ends up a very strange 9.5 x 15 feet (You lose 1/2 foot to the wall thickness) Not perfect, but not awful either.
Sorry Jay, I usually agree with you.
When laying out the bathroom, keep in mind you need to pay attention to wet walls (walls with the pipes in them) and making sure you can ventilate the drains to the roof, that the drains all have acceptable slope, etc. You can't just put drains any old place.