Hi there,
The secret to crown molding looking good is to make it a consistent feature in a room. You will find making the corners at the slope somewhat challenging to install correctly so take you time and buy enough to do a few test cuts.

One important tip for proper crown installation - if I may - determine how the crown will sit in the 90 degree corner between wall and ceiling then measure down from ceiling to bottom edge of crown and mark a line - painters tape works well without marring the walls. Keep that line as your reference so you avoid "rolling the crown" and changing the angle as you install it. This a common mistake even pros make and will complicate dramatically your efforts to make nice clean mitres at your corner joints.

Hope this helps,
Greg
Red Ridge Millwork & Design Ltd.
greg@redridgewinecellars.com
http://www.redridgewinecellars.com
http://www.allreclaimedwoodfurniture.com

I wouldn't use typical crown moulding for this room - It would never look right with all the angles, etc.

What I would do is install a simple flat 4" or 6" base moulding with a bead up at the ceiling, flipped over so that the bead is at the bottom:

http://www.ewmouldings.com/index.php/poplar-mouldings/base/poplar-base-board-ewbb26-p.html

That would be far simpler to install and appear appropriate to the style of the room - be aware tho that your installer is going to have to use little angled bits at the lower corners to transition from the full depth along the side walls to the angled walls to account for the differences in width on the angle...
...so hire an expert finish carpenter who is thoughtful, meticulous and understands the unique geometry of the space - Otherwise you may be disappointed in the results.