Hi. In RetroArch's video settings aspect ratio is set to core provided but sometimes I'm not happy with core options (or lack of option).
for example: I get too thin picture for PC-Engine CD (or Turbo Duo or TurboGrafx CD however you want to call it) because there is no option for aspect ratio in its core settings.
If I set aspect ratio in video settings to 4:3 I get what I want but that ruins aspect ratio for other cores which don't use 4:3 aspect ratio unless I change it back to core provided again. So I have to change AR in video settings every time I want to play Castlevania: Rondo of Blood and again for something else.