I am not aware of any way to do this. I believe it automatically takes the snapshot at the source's resolution.
What you can do is simply take the snapshot, then use something like Paint or Paint.Net and easily crop it. That's what I do and it takes me a few seconds.
It is. Go to your Preferences, and at the bottom it will say "Show Settings" and two radio buttons. Click on "All." Type in "snapshot" in the search bar at the top, and "Video" should be one of three results. Click on it and look for "Snapshot," then...