Particularly since you are just talking subject acceleration, I don't think that the size issue is a huge deal. There really is a huge range in sizes among children in the same grade anyhow. However, I have to admit that size was a consideration in not grade-skipping DS back in 2nd grade and choosing math subject acceleration instead. The sibling issue is more difficult because your younger DD may end up in class with your older DD. For us, I think that removing DS from the same math class as his twin sister was actually a good choice for both even though it was another confirmation that one twin was much stronger in math than the other. However, if you look at it another way, they need to find out sometime so perhaps it might as well be now.
