I don't think that your 12k is anywhere near the upper limit.
I am getting unexpectedly good performance from a 45k size page.
I have a page that is only two months old. We have been adding content and it grew to over 45k. Our techie has been away and we have been unable to split the page into several seperate pages, as we should.
Over the past two dances this page has come up out of obscurity to first page serp positions for a host of unintended keyword searches, as well as the primary keywords. I entertain myself by testing new phrases and finding our site near the top.
While I know that we will do even better by splitting the page and optimizing for sub-topics, the current situation is pretty good.
On a related note - A page of pure text is working well. Is there a new element in the algo that favours this?
We recently put up a page targetted at potential clients (not customers) that is nothing but text, a full page of text. The idea was to have a "reader ad" for potential clients wanting lots of reassurance. I did not plan for this page to be of any interest for the SE's or customers. The page jumped into high positions with freshie, and held them through a deep crawl. I am embarrased to note that it is outperforming pages that were carefully SEO'd for customers. I later went back and put some links into the text to better guide customer visitors to the part of the site that was intended for them.
If customers want come into our store through the trade entrance, it's fine with me. We will dress up the back door. What conclusions should I draw from the success of a page jammed completely full of text? Is this a coincidence, old news, new algo?