The photo above was shot with my Nikon D800 with a 300mm f/2.8 lens attached. What's significant for me, is this photo was shot as a jpeg image at a setting less than the D800's maximum image size capability. Until this photo, I've been shooting NEF files (Nikon's RAW file setting) producing 36 megapixel photos and files sizes exceeding 40 megabytes at times. With this photo I set the D800's image size setting to Medium which produces a 20 megapixel image out of the camera and only about a 6 megabyte file size. What I've discovered is a very acceptable level of image resolution which gives me enough resolution room to crop images and still maintain an acceptable quality of the cropped image. This photo was cropped from the 20 megapixel original to the 3 megapixel image you see above.
Jpeg image size setting for the D800