Download Avidemux:
http://avidemux.sourceforge.net/
Install Avidemux.
Open Avidemux.
Open the FLV video.
Choose MPEG-4 AVC in Video.
Choose MP3 in Audio.
Go to the Video Filters -> Transform -> Resize -> Choose the "200%" and "Round to the Nearest Multiple of 16" options
Save your video as the format you want. In this case AVI.
And that's all.
This is one of the easiest most safe methods so give it a try.
So here's my question:
Have any other ideas?

Regards.