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Select a youtube resolution and resize the player.
You can find a lot of things just by digging around with developer tools. For example, "ytplayer.web_player_context_config.rootElementId" in your javascript console will give you the ID of that element. But most of that stuff they could just change at any moment, so its best to avoid relying on their internals as much as possible.
Dude, how did you find out that the player html element provides the api to control the player? I'd never thought youtube would do that. Good stuff.