Here is a direct, side-by-side comparison of the apparent size of Comet Holmes and the Moon on the night of November 18, 2007. Both images are shown here at identical image scales for a true 1:1 comparison.

Based on this image we can get a rough estimate of the actual size of Comet Holmes. First, we know that the apparent diameter of the Moon and Sun are approximately the same, that is 30 arc-minutes. Knowing that the Sun is about 93 million miles (1 AU) from Earth and Comet Holmes was 1.64 AU's from Earth on the date this image was taken, the calculated "diameter" of the comet is a staggering 1.43 million miles!