With this function you can change (or set) the width of any room in your game except the current one.
Argument Description index The index of the room to set the width of. w The new width of the room in pixels.
global.myroom = room_add();
This will create a new room and store its index in the variable "global.myroom". It will then set its width to 640 pixels, its height to 480 pixels, its caption to 'Game Room' and its persistence to 'false'.