Well you can use
OS::set_window_size
to set the window size but I dont know what would happen if you tried to set it too a larger size than the current OS setup. You could use something like the below.
func _ready():
When something is true:
Resize_Screen(640, 480)
func Resize_Screen(First, Second):
OS.set_window_size(Vector2(First, Second))
Which would allow you to set the window size to anything you passed as arguments.