Well, inside of GameMaker you can use window_get_width() and _height() - in HTML5 they will get the game's available space in the browser, meaning if your game is not placed among other content they will return the full browser area. If you want to go completely fullscreen I believe you can use the window_set_fullscreen() function in HTML5 as well
Outside of GameMaker I know very little about, other than you can probably wrap the game in a div, which I guess you can make to fill up the whole screen (or at least the browser).
I am sorry I cannot be of more help, but I only recently started doing HTML5 stuff myself ^^