Font 7 is way too small and font 8 is not very readable. Font 9 and 10 should be fine; Preferably a 10 font and maybe 6 x 9 or 7 x 10. Let's see what other members think.
I own a single volume Bible that has 9 font size and roughly 1800 pages in a 5.5 x 8 size. I think a 10 point size would fit into a 6 x 9 with roughly the same number of pages.
For to me, to live is Christ; and to die is gain (Phil 1:21)