All of these settings can be rather subjective, so don't be afraid to play around with them. I´m going top to bottom.
Height / Width. No less than 384x512 No larger than 768x768 -> Larger images can cause multiple people to appear. Use upscaler to make your images larger. Image Scale
Example of weird results. I did not cut this image, thats how it was genned.
Model: Pick one of the UMI models from here. (They should already have been downloaded with Artroom) Models