I just went through the process of creating a new cross-platform desktop project and transferring my current project files over to it. For some reason, the window is stuck at the initial 800 by 480 resolution. PreferredBackBufferWidth/Height does nothing, and directly setting GraphicsDevice.PresentationParameters.BackBufferWidth/Height, even in the update loop, does nothing either.
To check, I created a blank new cross-platform project and confirmed that, even in a blank project, PreferredBackBufferWidth/Height is ignored.
Edit: Actually, I’m not even sure what version of Monogame the project is using. Presumably just the version that the template extension grabbed from Nuget when I created the project, if that’s how it works.