Image download files can be purchased in four sizes:

Web (640x480 up to 0.3 Mpix),

Low-Res (1 Mpix)

Hi-Res (4 Mpix)

Original (varies)

Original image sizes range from approximately 4800 to 10800 pixels on the long side of the image at a resolution of 300 ppi.  They are considerably larger than the preview images you see on the site.  The example below shows the difference between a detail section of the largest watermarked Preview image (right) and the full size Original image available for purchase (left).