Search and Download WinFF.exe, its configured to work with RockBox.
WinFF lets you convert most vids to MPEG's for RockBox.
And no it will not be out of porpotion. When choosing your System
on WinFF, go to RockBox and Set to iPod Nano 4:3 and Convert.
CMD will pop up, let it do its dang thang and done. then browse to where ever you saved the Converted VID and create a folder in you iPod Called "Videos"/"My Videos" and place them there. Start RockBox and go to FILES then Videos. Play. Hope this Helps...