Well, yes, you can add a rectangle quite easily:

    type Shape =
        | Circle of float
        | Square of float
        | Rectangle of float * float

The thought about adding new shapes (mentioned several times in this thread) is just misplaced. If you're releasing library code people should not be extending your built-in types, that's bad form -- consumers of a library should interact with it through interfaces only.

The flipside is that its very easy for your library users write a new function that takes a Shape as a parameter. If you model things with a class hierarchy its easy to add a new Shape subclass but you can't add any new methods to the Shape interface.

That comment about shapes with more than one defining size property is very valid.