In my experience I've found that introducing new chickens to the flock is a lot easier if you add four or more at once. Then the newbies can hang out together, and present more of a united front to the older chickens. Could you get a few more chickens from wherever you got the new one so there's more than one of her size? Also, when introducing any new chicken (of any age), it's best if you can keep them in a separate cage where they are safe from the existing flock but still in sight of them. That way, the old flock can get used to the presence of the new one. If she is still getting picked on, I would make her a separate space where they can all see each other but she's protected. And I would really try to get a more more chickens that are her same size....
