Q:

Looking for a word meaning small/petite

Help I’m drawing a blank!  
Looking for a word that describes a person as having a small build and also fits into the sentence “she was a _______ of a woman”

A:

Oh yes, I believe you are looking for "wisp" as in, "a wisp of a girl", or in the case of your sentence, woman. 
Merriam-Webster's definition here.