I've been printing (and selling hundreds of large prints and canvas) printed from images captured from 4, 6, 10 and 12 MB Nikon captures, most in the 20x30 (50x75cm) to 30x40 (75x102cm) sizes. At a foot away, you can see they are not perfect but at proper viewing distances 3+ feet away (1 meter plus), they are superb and my bank account is very happy as are my clients!
The only way to see how large a file from a D800E can be printed is to simply do it yourself and see what you think. I'd say for me with my D800 and good craft, I could easily print 40x60 (1x1.5 m) or perhaps even larger. In my spare time, I'll be giving it a whirl to see how one looks.
Factors to consider include what the best file res is for your printer versus what res it actually prints (many printers in reality can work with much smaller files than most realize and probably have never tested).
A good way to give this a try is to take your typical image file and make an 8x10 print (20x25 cm) of that image from several different resolutions--say 50 dpi, 100 dpi, 150 dpi, 300 dpi, 360 dpi, 600 dpi. At a normal viewing distance, say arm's length, put them up on the wall and see what looks best to you. Simple to do and a great exercise if you haven't done this before.
A good real-world example of this exercise was conducted by tech guru David Pogue in 2006:
http://pogue.blogs.nytimes.com/2006/11/21/21pogues-posts-2/ Only one person in 100 that tried could determine which of three prints was which from the different input resolutions.
Repeating a similr test myself, I've found how far I can push my own images. The math will never agree with my results;-)
