Other options include 8.5x11 (which is much too large to be easy to hold or easy to lay out).
6.625x10.25, which is an interesting idea, gives it a more horizontal feel and slightly larger than 6x9.
9x7 Landscape which would give it a very hoiztonal feel
or a 7.5x7.5 square... which would give it a very .. er.. square look.
Personally I am debating between 6x9 and 7.5x7.5. Any opinions out there on what size they would prefer?