I followed the instructions here at the bottom (Using MediaCoder to encode on Windows )
http://www.rockbox.org/twiki/bin/view/Main/PluginMpegplayerAnd the resulting file is 352 pixels x 288 pixels when I select the sansa with the extension applet. I was wondering if this is by design, or if this is an oversight. Also it seems to me that changing the video bitrate (using the extension applet) doesn't change anything in the resulting file. I have no way to edit (or look at the applet apparently) so I can't figure out what the presets are or where they are stored.
I wasn't sure if there is a problem with the rockbox extension, or if this is just the way it was designed.
I have used super to make files 240 x 176 that play fine and are smaller in size, so I know it isn't a limitation of the sansa itself.
Any thought would help, meantime I will experiment.
There is also an "extension missing to display all media on theis page" error and when I let it look for the extension, it is not found. I mention this to see if it helps.
EDIT: It seems that there is a setting in "picture" with a checkbox for "global" and the 352x288 was selected by default. I needed to change the settings in the main window to get the proper size, the extension would not do it by itself.