I own a 8" and a 12" steel. Honestly i wish i had a 10" the 12" is much bigger and harder to hold with my style of use. If you plan on using the way directions tell you to, Holding end against cutting board and sliding knife downward. I always hold mine in air, with my 12' it just seems more difficult but that's probably because i gave up on getting better at it and use my 8"
Which I only own a 10" and a 8" chef knife( my longest). both wusthof classic ikon series
So i would say the 10' would be great especially if she is petite, some of the 12' are around 1+ inch thick and could be more difficult to handle with ease. ( my opinion and experience with them )
Hope it helps i'm sure she will enjoy it! I say keep the one you got!