There’s a long-standing bug in Fedora (also in Fedora 40), where the sign-in screen is displayed at the wrong resolution (it doesn’t use the settings you have set in the Settings app).
So, for example, if you have 4K@2x set in your settings for your monitor, the sign-in screen will still display at a tiny 4K@1x.
To fix, download an app called GDM Settings from the Software app and press Display → Apply current display settings → Apply.
