No need for all this! You should use only one collision box.
The manual says that because changing the box too much may cause issues like going through walls and that messes all the engine, but you can make it in a controlled way.
What I do is:
Make two animations for the collision box, one used for all sprite animations, and a shorter one for crouching and sliding. Make sure the origin is perfectly positioned so the box won't move when you change animations. I also think the best shape for it is a perfect rectangle.
Let the player crouch without limitations, but whenever your character gets up, there's a condition checking if its overlapping the tilemaps. I can be done this way:
-If player is not holding the crouch buttom: change the collision box animation to standing
-Collision box is overlapping tilemap: change the collision box animation back to crouching
There's no problem in forcing the animation to stand even when there's the possibility of it going through walls, as long as you change it to it's original state when it does. It's a good way for checking. :)