I have experienced the same problem following upgrading to the new version. This computer, the one I use for OpenLP, recently upgraded to Win11.
For some reason, OpenLP is mistaking my screen resolution and reporting it as much lower (960x540) when it should be 1920x1080.
My Windows setting has screen scaling set at the recommended 150%. I have changed that to 100% (BEFORE starting OpenLP) and that makes OpenLP report the screen at correct resolution, and all dialogs/content now fits on the primary monitor. Not sure about secondary monitor/projector yet, still checking ... I may uninstall and reinstall with screen scaling set to 100% and see if that forces OpenLP to correctly recognise screen size/resolution. I have tried changing screen scaling while OpenLP was running, and that did not have a good outcome, OpenLP just 'hung'.
I've only just discovered the influence of the screen scaling control and still experimenting ... It is a tad inconvenient using scaling at 100%, everything is very small and I need to use my glasses ...