Thanks mouser, I hope I'll do better the next time,
I will pursue working on it a little bit anyway, I've found a way to detect the size of the flash more reliably by reading the header of the swf. This is something that can't be done by simple extensions.
I would like to try to conserve the ratio of games, it's not always correct depending of the resolution of the game (I have no solution for this right now )
I will also make extension package instead of an installer, so that I could upload them to extension websites, and it could be a little bit easier to install.