9 foot ceiling should have about a 4" crown. Classical and traditional architecture never had these "giant" crowns we have now, they used to increase height by doing build-ups of moldings that also increased the moldings projection into the room. Tall, one-piece crowns wind up appearing very flat and upright. If you can squeeze in a picture rail about 4-6" below the crown and paint the wall in between the two the same color as the moldings, you will nicely emulate a traditional entablature.

like this:

Thank you Artisan Carpentry - the picture of the molding you sent is the perfect scale. I will take your advice and avoid the bigger is better molding.

Just wanted to second what Artisan stated. I would go with 5" as well. Whenever I see those huge crowns, it always distracts me and in some cases can make the room feel smaller. Good luck with your project. Charmean Neithart

glad to help- sometimes this stuff is not intuitive. I like what Charmean said about being distracted and the room feeling smaller

Something I like to do with crown molding is to paint it the same color and finish, usually flat, as the ceiling. This gives the impression that it is not painted wood, but in fact molded plaster, which is far more elegant a treatment.