Solution
Smplayer > Preferences > Advanced > Options for Mplayer > Options > add
-aspect 16:9.
Lubuntu 20.04
But sadly Smplayer is pretty useless for me as it fails to open files.
http://ubuntuforums.org/showthread.php?t=1790983
Read posts 1 -4. The later posts deal with a different issue which is now solved by installing mplayer2.
Lubuntu 20.04
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