Right now, when you open a page with an embedded HTML5 game that has the "Automatically start on page load" option disabled (i.e. a game that requires a click to play), the "click to play" screen shows at the size of the embedded game.
In contrast, if you open a page of an HTML5 game that runs in fullscreen ("Click to launch in fullscreen"), the "click to play" screen shows at a small size.
My suggestion: allow creators to use the small size for the "click to play" screen, even for not-fullscreen games. The canvas would then resize to the correct size when the game is run.
The reason for this suggestion is that sometimes, the embed is simply too big. I always fear that someone may actually think that there's nothing else in the page other than the playable game. With a smaller embed, it's obvious that there's information under the game area.