fullscreen_width
The option documentation is properly formatted yet
Example: 1024 Since: 1.1.8
Use to override fullscreen mode width. Normally this is not necessary, as FS-UAE uses the current desktop resolution by default (recommended), but can be useful in some circumstances (multi-monitor setups on nVIDIA/Linux).
Source: https://github.com/FrodeSolheim/fs-uae/blob/master/docs/options/fullscreen_width