Any size you like.
The key factor is that the bigger you print a picture, the further away you'll want to stand to look at it, and therefore the lower the resolution you can get away with.
On the
Cambridge In Colour web site, there's a resolution calculator. For somebody with 20/20 vision, it suggests:
350 ppi when viewing at 25cm
175 ppi when viewing at 50cm
87 ppi when viewing at 1m
18 ppi when viewing at 5m
(ppi = pixels per inch)
So with an image that's 13,000 by 6,000 pixels, you can have:
* 37" x 17" at 350 ppi - will look sharp even when viewed really closely, but it's too big to look at really closely
* 74" x 34" at 175 ppi - will look sharp at arms length, but it's too big to hold at arms length
* 148" x 68" (12 feet by 5½ feet!) at 87 ppi - will look sharp on the wall - but it's the size of a wall
You get the idea ...
Two key considerations though.
Firstly if the original image isn't sharp then none of this applies. That's why pros who want to make big images use really expensive lenses.
Secondly, I'm a bit suspicious about where that pixel count came from. 13,000 x 6,000 is 78 megapixels, and if it's a 6-shot panorama then that's 13 megapixels per image not allowing for any overlaps, or perhaps 20 megapixels per image allowing for overlaps. Your D200 doesn't give you 20 megapixels - have you enlarged it in PhotoShop? If so, they aren't "real" pixels and you won't achieve the sizes I've set out above.
If you can describe how you got to 13,000 x 6,000 I'll be able to help you estimate print sizes more accurately.