I love both
Rebecca and
Anastasia
I think
Lily is a bit short, the two syllables helps, but still short IF you're not counting nicknames
Because honestly, you'll end up with nicknames regardless;
Becky; Bex;
Becca and
Anna
Becky,
Anna and
Lily
Perhaps you could elongate
Lily?
Lilian or
Lilianna? so you still get
Lily, but also it don't look so weird being so short in comparison?