So, looking up
the rules, it appears that a winglet there would not be allowed, except that in article 3.4.2 it says
With the exception of a transparent windscreen, antennae, pitot tubes, the mirrors described in Article 3.5.2 (including their housings and mountings) and the high speed camera defined in Article 8.8.6, no bodywork situated between the planes A-A and a plane parallel to and 380mm forward of the plane C-C may lie more than 625mm above the reference plane.
So this comes down to "..housing and mounting" (from the bit I bolded in the above). Ferrari are hanging the mirror from the winglet to give support to the outer edge of the mirror and stop it wobbling and so will argue that it is "mounting" for the mirror, that's allowed provided it meets this confusingly worded set of restrictions (from 3.5.2)
Other than the rear view mirrors (including their housings and mountings), each with a maximum area of 20000mm² and 14000 mm² when viewed from directly above or directly from the side respectively, no bodywork situated more than 330mm behind the front wheel centre line and more than 330mm forward of the rear wheel centre line, which is more than 600mm above the reference plane, may be more than 350mm from the car centre plane.
where I think(?) only the bolded bit applies to the mirror size. Because the mirrors are fairly narrow from the side, and the winglet sits directly above the mirror from above I guess it meets these requirements.