From YoYoGames Wiki
(real): the width of the current room
Important note: this variable is broken in HTML5, and will always return 0.
This variable holds the width of the entire room (you need to use view_wview to get the size of the view inside the room). This variable is read-only, you cannot use this function to change the size of the room.
// Create an object at the right hand side of the room instance_create(room_width,100,obj_player);
In the example above, as you're using room_width, no matter what size the room is, the object will always be placed at the right-hand side. You can also use room_width to center objects on screen. First, make sure the origins of the object is in the center then you divide room_width by 2:
// Create and center the object horizontally in the room instance_create(room_width/2,100,obj_player);