Alexandra is already inconveniently long and is inevitably shortened -
Alex and variants now,
Sandra back in the 40s and 50s. And
Alexandria adds yet another syllable, while converting itself to a geographical name as well. And I don't enjoy geographical names, or the nns for Alexandr(i)a. In the case of
Alexander, I like
Alec as a nn but it is seldom used, and in my experience never on a girl.