Using
Alexandra does take out a syllable, but it is still very long itself. Also using another full name as a NN for a similar full name seems odd to me. I would be confused if I met a girl called
Alexandra and she later told me that her name was actually
Alexandria. Seems to me that if you're going to say the 4 syllables of
Alexandra, you might as well throw in the 5th that makes it
Alexandria.
How about:
AlexAlexa (a little like using
Alexandra as a NN, but since it's significantly shorter it doesn't seem as odd)
Lex/Lexie/Lexa
AliZan (Xan?)
Xandria
Andie
AndriaAndrie
Xandie
Xandrie