Sadly, you will have to wait a long time.
As far as I know, there have been no announcements of a larger than 9.7 e-ink device for sale.
In fact, there is a veritable dearth of 9.7 devices. Basically, you have the 3 year old Kindle DX, then there is Pocketbook 9xx series and Onyx M92, oh and the dismal Jetbook Color.
I suppose you can order 12 inch developer kit from eINK but it must cost at least a few thousand bucks.
http://www.eink.com/concept_showcase_12inch_reader.html