keep_aspect
The option documentation is properly formatted yet
Description: "Keep aspect ratio when scaling (do not stretch)" Default: 0 Example: 1 Type: boolean
If keep_aspect is set to 1, the amiga video output will be scaled with the aspect ratio intact, otherwise it will be stretched to fill the screen.
Source: https://github.com/FrodeSolheim/fs-uae/blob/master/docs/options/keep_aspect