If the image is big enough, and the right aspect ratio, it will display as close as possible to full size without zooming. The optimal size for the Kindle 2 is 520px by 622px, it might be slightly different for the K3 but this is probably still a good size to use. See
Kindle Maximum Image Size.
The other approach would be to include the image at 600x800, or whatever works best when "zoomed" to full screen. The Kindle will automatically shrink this to fit in the 520x622 available in non-zoom mode (it will maintain the image's aspect ratio).
EDIT: If you want exact control, see
Is there a "fix" or work-around for Kindle doubleing size of small graphics? This does not allow you to exceed 530x622 though.