My DS7 liked them at age 4 and 5, IIRC. He re-read them as quick and easy. At 7, looking at what he's reading now (The Number Devil), I don't think he has a preference for size of books. But a couple of years ago, when the MTB were an interest, the size was an issue. He would happily read through the smaller regular series books in a flash, but was more hesitant when it came to the bigger Merlin Mission books. So, for that type of book, I'd vote for series of smaller.
Congrats on finishing your Nano novel!
