The best advice you have received so far is "Go to the store (the both of you) and hold them in your hand." I have a 12" Shun Chef's knive. Way to big for me so I gave it to my son. He has no problem holding it. What knife feels comfortable for a woman may have a handle that is to short and thin for a man's hand. Don't be afraid to buy two of the same knife, one for her, and one for you. If she can't comfortably control the knife, she will not be willing to use it.
Illegitimi non carborundum!
I don't want my last words to be, "I wish I had spent more time doing housework"