I wouldn't use any nickname for it. I dislike
Chris, it is so rough and unattractive, even on a man.
Chrissy and
Christy are so 70s. I also dislike
Ana and it doesn't work as a nickname because
Christiana is kris-TYAH-na, at least where I live, and
Ana is pronounced differently. Maybe Iana or
Yana (YAH-na) would work but I really think it doesn't need a nickname. It only has three syllables plus I don't even think
Anastasia needs a nickname and that has five syllables where I live :P
I know two very nice girls with this name (actually I know three, but I don't know the third one very well). Two spell it
Cristiana, the other
Christiana. The two Cristianas are from Romania and Italy and
Christiana is from Germany but lives in Austria now.
I like the name, it is classic, feminine and pretty. It is not dated, like
Christina or
Christine.
A short middle name, definitely:
Christiana RoseChristiana EliseChristiana BelleChristiana WillowChristiana YasmineChristiana Noelle or
JoelleChristiana Eve