R.,
Your request brought back so many memories for me! When I was 10 my dad built me a dollhouse, a 3 story victorian with a porch and dormer windows, I loved that thing! I continued to enjoy/play with it well into my teen years.
I would build it regular doll house size, when I was younger I had some small "doll house people" who lived in the house, as I became older I just enjoyed getting new furniture for it, making new curtians, decorating it, etc.
Something to think about is wouldn't a barbie sized doll house would take up a lot of room? Anyway I'm sure its something she will treasure forever. I had 5 boys who were not the least bit interested in it, and it is now on "loan" to a very good friend of my mothers for her granddaughters to enjoy.