If you have trouble maximizing those YouTube videos when run under compiz-fusion, here's what you can do to fix it.
Go to CompizConfigSettingsManager, choose General Options and unclick 'Unredirect Fullscreen Windows' option under General tab. Then click Back and select 'Workarounds'. Turn on 'Legacy Fullscreen Support'. Of course, 'Workarounds' plugin must be enabled on the front page for this to come in effect.
And that should be it!
Now try maximizing flash videos again and report you success below.