Is using a USB flash drive or an external hard drive an option to play the Despecialized Editions?
It may save you the hassle of resizing files to find an acceptable image quality for you - and also burning discs etc?
If not, and you want to go the disc route, then these may be of some help:-
https://www.quora.com/How-do-I-convert-mkv-video-files-to-mp4-without-losing-quality
https://www.freemake.com/how_to/how_to_convert_mkv_to_avi_mp4
https://www.tipard.com/mkv-video-converter/how-to-convert-mkv-to-mp4-with-mkv-to-mp4-converter.html