I'm not sure I'm understanding you right, but I'd suggest you perform a test with JPG set to Medium, Fine compression and Quality Priority. The will be smaller in both pixels and file size than the D600, but still much bigger than the needs for 99% of people.
If that does not meet your needs try Large, Normal compression, and Quailty priority. The JPGs will have pixel dimension larger than the D600 image at the largest size, but a smaller file size than a D600 Large, Fine image - I think!
For the D800 you have three sizes to choose from Large - 7360 x 5520, Medium - 5520 x 3680, and Small - 3680 x 2456
And choose the compression ratio you want for the size JPG Fine - 4:1 compression, 16-18 MB for large size JPG Normal - 8:1 compression, 9.5-12 MB for large size JPG Basic - 16:1 compression, 4.8-5.5 MP for large size
In each one of those you can use a Quality Priority providing more quality or the Size Priority which will produce a file that may not have the high quality of the Quality Priority.
You also have the choice of shooting in a crop mode, 5:4, 1.2X, and DX. And through each of those you can again choose Large, Medium,or Small size as well as Fine, Normal, or Basic compression.
There are simply enough options here to confuse most anybody! I did try the crop sizes - but can't get used to staying within the lines in the viewfinder.