I understood you first post, but a libretro core might not read or write the same .cfg file as the standalone emulator. Usually when an emulator is ported to the libretro api, its options are implemented and exposed as Core Options in RetroArch.

Ooops, sorry. Wrong thread.

@RedFarmer i have trouble with the 1280x1024... the game field aspect ratio is not right... the cfg is not working original resolution was 160x144px which is 1:1.111 but your version is is almost 1:1.

I think ill try a different one or convert one 16:9 myself

@mitu thanks for the tip! Its definitely useful info if there will be some problems with positioning image on screen, Ill save this video for later use.
Actually I found another solution for the problem:
before the game starts there is a grey info box, you can push some button and then enter a menu with additional setting. From there you can enter default video resolution submenu and play a little with setting that are stored there. I managed to fit image to the screen at 800x600 4:3 setting. It works with both psx and snes emulation. Maybe this info also would be usefull for someone.
Thank again and good luck!

@Garisaan i have done something sort of like this here: https://retropie.org.uk/forum/topic/4046/crt-pi-shader-users-reduce-scaling-artifacts-with-these-configs-in-lr-mame2003-lr-fbalpha-lr-nestopia-and-more-to-come

if you use the last two links in that post, it will give you crt-pi-curvature-vertical shader in vertical mame games, and the regular curvature shader in the rest.

Sorry just saw a dedicated thread by @meleu about this very thing.

Please close :)

@ruckage :Oh my god! You just made my entire month!

@dankcushions Thanks for the tip, I changed the setting to 5:4via the Configuration Editor, that then made the changes to the retroarch.cfg file and that's now the default when I load games. Interesting that by changing it manually it did not work. Anyway, thanks again for all the help over the past few days. I hope this thread is useful for someone else one day who also wants to use a CRT and really wants fullscreen :)