Somehow, I came to the conclusion that the 1.8" display on my player was 160x128, and that 160x120 was the biggest size I could play. Trying mpxconverter from
http://www.bytessence.com/bmpxc.html has led me to the conclusion that it is in fact 192x160, and 16fps videos at this resolution plays fine, filling the screen. This is probably the native resolution of my display, some other 1.8" displays use 176x144 instead.
Perhaps some other users have made a similar error and could benefit from a little experimentation to find the highest resolution their players can use.