Actually, all Justin.tv streams are 4x3. Some are just displayed letterboxed. The fact that they were fullscreen previously was actually a Roku bug that was fixed in the latest firmware. I'm looking into adding a zoom feature in the next version.
As for the FilmOn channel, all of those should be fullscreen unless the stream is 4x3. Make sure you have the latest (1158) firmware by going to Settings->Player Info. It could be that you're experiencing the squeezed video bug that they just fixed in the new 2.8 release.
My Channels: http://roku.permanence.com - Twitter: @TheEndlessDev
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