On page 173 of the manual they mention 31x (Large Image), 23x (medium Image) and 15x (Small Image.) I seem to remember discussions that came to the conclusion that the medium size was a 100% (one image pixel represented by one screen pixel) on the LCD.
I do think that 100% is plenty enough to judge your focus during a shoot. Remember that anytime your process an image for printing, especially if you resize the image, you should apply sharpening.
In fact the image you are reviewing on screen is a JEPG image created from your RAW data that has all of the picture controls applied, including sharpening. So if you are using non-Nikon software the image may not look as sharp on your computer until you apply capture sharpening. If you use Nikon software then the picture will look as it did on the LCD.
Dave Summers Lowden, Iowa Nikonians Photo Contest Director
Nikonians membership - "My most important photographic investment, after the camera"