You have two options. If you add a CollisionShape2D child, you can add your new shape to it. Otherwise, you have to use the collision/shape_owner API, which I think is more cumbersome to use.
var collision = CollisionShape2D.new()
var shape = RectangleShape2D.new()
shape.extents = Vector2(5 * PlatformSize, 1))
collision.shape = shape
See CollisionObject2D docs for details of the
shape_owner API. You might also find it easier if you create a default platform scene that includes a collision shape and instance that, so that you only have to set the shape extents.
You didn't mention what error
add_child() was giving you, so I'm not sure what to advise about that.