window.get_size() - This returns the current window size (width and height).
So when you resize your window it will change the width and height of the window to different dimensions depending on how you resized your window. You can resize the game window with both html5 and desktop applications but may get different initial results depending on what you are building for.
For html5 it will generally give the viewport of the browser which is usually set as full screen of the users monitor.
When building the desktop application it is set more closely to the display settings in the game project settings because you have more control with a desktop application versus web development in how it is displayed to the end user.