Capcom's CPS2 games use 384x224 resolution, which is technically a 16:10 resolution but squished into a 4:3 frame. The closest resolution the Saturn has to that is 352x224. That's what the Capcom CPS2 ports all use with the original arcade assets. Lowering the resolution won't make the characters slimmer, it will just make them fatter. Going up to 640 wide wont help either. So 352x224 is the best option available on the Saturn as you can use the arcade assets as is and still get pretty close. If you want slimmer characters on Saturn's resolutions you would have to redraw all the assets for that resolution.
