I started out in 8.5x11, but was never really happy with the results. When I switched to 12x12-It just worked! My techniques have improved and I feel a lot more confident about the end results.
I did a 6x6-That was rough! I had a very hard time getting it to work. If I want to do a small one again, I would go with an 8x8. I had pages that were 6x6, which is why I went that route. I love the book, but it was way too hard for me to do one that small again. (Although I think if I'd had the pics printed as 3x5s instead of 4x6s it would have been easier)