ProjectSettings.get_setting("display/window/size/width") to get base width,
ProjectSettings.get_setting("display/window/size/height") to get base height.
To get current window size, you need to use
OS.window_size (note that this will return Vector2).
Then "resize value" will be
base_width / OS.window_size.x or
base_height / OS.window_size.y based on your situation (note that users can resize window to different aspect ratio, so you probably will have to use smallest value between width and height to get correct "resize" value).
If you need to know when window is resized, you can connect to
screen_resized signal of SceneTree (
get_tree().connect("screen_resized", self, "_on_screen_resized")).