This is by design. First, we expect HTML5 games to go onto websites, and as such they will have web pages designed around them, or they will be placed into spaces set aside for them. For this reason, we believe having the canvas changing size and breaking web page layouts is a bad idea. We have also taken the choice to allow scaling of the game by changing the index file. This is to allow better integration with web sites, in particular the various games portals which may have very specific requirements for sizes of games. This allows you (or them) to change the size of the final game, without having the source, or needing to recompile the actual game. Lastly... changing the size of the game play area, is a terrible user experience, and one which will be going away as we go forward, so it's something you'll need to get used to.
You can change the size ofthe canvas, but you have to do in MANUALLY. See this GMC topic about resizing.
Changing rooms does not change the size of the screen/canvas
