Perhaps request that both your DDs be placed in the same homeroom and a couple of the key courses. In our system, a lot of the administrative stuff gets taken care of during homeroom, which is the first 10 minutes and last 5 minutes of the day.
I agree with the suggestion not to push for placement with her best pal. It will be easier for your DD to branch out and make new friends if she doesn't have that kind of clutch. It is important to make more than one friend in every class since it is not uncommon to miss classes here and there for rehearsals, field trips, etc. Although I have twins, they only share a couple of classes so their friends have been great sources for stuff they missed or forgot to write down. They generally get a quicker response when they im or skype or text their friends than their teachers.
I would also have your DD examine her interests and narrow down some clubs to join as that is a fantastic way to make friends, especially for kids not in the same classes this year but may well be in the same classes next year.
I really would not worry so much about her size. If she is already 10 now, there will be plenty of kids who are within 6 months of her in age in 6th grade. Due to different growth spurts, kids really vary in size in middle school. There are kids under 4 and a half feet as well as kids over 6 feet.
