Machinima Monday is a group in SecondLife tath meets most Mondays at 3pm SLT. Lots of folks there, all interested in making movies using platforms like SecondLife and World of Warcraft. People show off current projects and ask for help. Sometimes Chantal Harvey puts updates about the group on her blog - http://mamachinima.blogspot.com/
My machinima buddy asked yesterday and Fraps and screen size. We learned that some software like Fraps and Xfire (both PC products that record the screen) record the whole screen. There's a preference you can set in SecondLife to make SL run in a window and hten you can set the size and resolution. You can pick one that's a 16:9 ratio when you're recording in HD.
On the Mac I use Snapz and i can draw a box with that software and it records only what's in that box. I haven't played around with that much. Not sure what it looks like in playback if you record just a little box recorded.