You change the scale variable. It's a vector2 or Vector3 depending on the node it's on.(Vector2 for 2D; Vector3 for 3D)
It would look something like this:
func _on_area_entered(area):
scale *= 2 #this doubles the size of the node it's attached to.
func _on_area_exited(area):
scale /= 2 #this halves the size of attached node.
Hope this helps!