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