Hello,

You can specify its height and width in height/width property. To set maximize for window, simply change WindowState property to Maximize. Last about window startup position, you can configure the position in WindowStartupLocation property.

Regards,
Handy

The window point to ApplicationUri is a uxpage,not a uxwindow,there is no  WindowState  and WindowStartupLocation property.

Hello,

Yes, I know. Normally, UXPage is always in Maximum size. Perhaps, you container which loads UXPage is not maximize. 
E.g In our ClientUIDesktopApplication template, Welcome.xaml is a UXPage which load inside a window.
As default, it always show in full container. You can define the loaded window/container just from its window/container property. If you use window as its container, it surely has properties that I mentioned before.

But if it is purely UXPage itself, the position can be set WindowLeft and WindowTop properties.

Regards,
Handy