I feel as if I've said this in another thread, but size is mostly a matter of preference. We should each do what seems right to us.
Personally, I'm a 6 X 9 guy. Aside from one book (which no longer has a paperback), the works I have in paperback are all long enough that the size doesn't seem odd to me.
I've had a lot of experience with print books. I've accumulated at least seven thousand of them--don't judge!--and I have them shelved mostly by size to avoid wasting space. In the old days when I still had room and used to reshelf once a year to incorporate new purchases, it was hard not to notice that 6 X 9 was by far the most common size. It is true that the total includes various genres, including nonfiction, as well as hard covers, Still, 6 X 9 was big even in the genre in which I write. The second biggest were the mass-market paperbacks of old, which were smaller. In the days in which I bought a lot of them, they were literally pocket books--they fit in your pocket.
It's also worth noting that the print costs are determined by number of pages, not size of pages. That means a slightly larger print size is slightly less expensive to produce.
