my experiences
I started with Lulu and had my novel published at the 6x9 size. I too would have rather had mass market size, since all the books I have are that size, but when I received the published book, it seemed fine by me. Then, I went with Createspace and at the time, they offered only 8.5 x 5.5 as the closest approximation. Since then, I like the smaller version a bit better, so I've been going with that size.
If you have a lot of pages, might be better to go with the larger size and if you have few pages (and want the book to be physically easier to hold; I had a poetry collection that was smaller re page number and benefited from going with a smaller page size), maybe go smaller.
Just my experience.
Michael