Not sure what I’m doing wrong, but RetroArch on my Wii will always reset the video resolution to 448 x 448 when starting up. I’ve manually changed it to 640 x 480 in the settings, saved the config, and made sure that configs were being read from the correct directory. When I save the config in-app and exit without restarting RetroArch and then read the cfg file on a PC, it shows that my custom resolution did indeed correctly save as 640 x 480 but as soon as the RetroArch app is launched on the Wii, it automatically overwrites the cfg and changes everything back to 448 x 448. What’s going on?
Are you trying the nightlies?, If so think devs made changes to the resolution setup process and it might be a little buggy for now… we need to wait until it gets fixed 
This happened while using a stable release. I updated to the latest nightly and it still happens.