force_aspect

The option documentation is properly formatted yet

Summary: Force Aspect Type: Double Default:

You can use this option to specify a forced output aspect ratio, e.g.

force_aspect = 1.3333333333333333

For a 4:3 aspect ratio. This aspect ratio will be enforced regardless of the size of the zoomed Amiga display. This option can be useful in combination with auto zoom to make the output size "stable", e.g. to combine auto zoom (which is really a crop function) with a fixed 4:3 display.

Source: https://github.com/FrodeSolheim/fs-uae/blob/master/docs/options/force_aspect