Both jgodfrey and _magicalogic_ are right: You can't get a "length of rotation" without specifying a radius. You can either get the angle with $My_Node.rotation
(like _magicalogic_ said), or get the length of the arc segment at a specific radius, like jgodfrey said. For the latter, there's no inbuilt function (as far as I'm aware), but you can easily create one yourself:
func get_length_of_rotation (angle: float, radius: float) -> float:
var circumference : float = 2.0 * PI * radius
var fraction : float = angle / (2.0 * PI)
return circumference / fraction