To do this (while supposing you're using a rectangleShape2D as your collision box) you can change the size of the given node with this code:
var screen_size = get_tree().get_root().get_rect().size
var shape = get_node("yourCollisionShape2D").get_shape()
shape.set_extents(Vector2(screen_size.x * 2, screen_size.y * 2))
According to the documentation, you have to use twice the size of the rect you want to use, which is why both values are multiplied by two...