MAUI for Windows runs on WinUI not UWP, so UWP methods aren't applicable. WinUI gives greater access to native window and window management API.
The WinUI way to set the page size is to manage the WinUI AppWindow. From a MAUI app you can do this when the window is created by managing the OnWindowCreated lifecycle event.
There is a discussion of this with unofficial sample code in the MAUI repo at: https://github.com/dotnet/maui/discussions/2370 . Even though the poster's original goal was for MAUI Blazor+WinUI, there is nothing Blazor related in the answer. This all happens in the MAUI + WinUI frame outside of the Blazor control.