Not sure you need an app to do that. Just run AirPlay to the apple TV, then once the desktop shows up, start the youtube video in whatever size you want (all the way up to full screen) and it should play just fine. Or are you trying to accomplish something else?
If you are having some problems getting Airplay to work with your Mac you may find AirParrot2 useful. I'm using it to stream content from my 2008 Mac Book Pro to an Apple TV.
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