I modified frododvd.ini to run in 800x600. It works fine standalone.
However, when I run it within MediaCar with a 800x600 skin, the FrodoDVD window stretches to 848xsomething: its menu bar is stretched and cut at 800 pixels (half of the "volume up" button), and the movie frame is offset 38 pixels to the right, then cut.
In MediaCar, I run it embedded with the string *FrodoDVD*. Same problem without embedding. I also tried after deleting all skins except 800x600 in the FrodoDVD directory. No success.
Again, FrodoDVD alone works fine.
Edited: found that external_dvd.skin was wrong, still configured for the default 848x480 skin. It works now.