The 2x16 plate and the 6x10 plate can be replaced by shorter ones, if necessary.
The plates covering and concealing the hinge prevent the door from opening in the direction of the studs, and the piece on the swivel keeps it from being pushed away, until it is turned with one of the keys from the Elves sets (or a Technic axle).
You can decorate the wall however you like. Slope bricks create the appearance of a rough-hewn tunnel, as in the last post, and smooth plates make it look like a wall. You can add basically anything as decoration:
It would be fairly easy to make the wall taller so that the door is larger, but if the walls with secret doors are taller than all of the other walls you use, it will be obvious that something is going on.
If you want this to be placed on a Lego baseplate, then it becomes a much harder build. An experienced Lego builder could figure something out, but it is a lot easier to set the terrain for your games by placing walls on the mat or grid.