NASA gives lowball estimate of 25 miles in diameter (at the smallest) but gee, it looks a little bigger.

Hale-Bopp on October 18, 1996. The blue dot is not someone testifying against a Kennedy, the Clinton's, or O.J., it's how big the earth would look if it were next to the comet. This picture is a false color picture of Hale-Bopp I took when it was about 280 million miles from earth. The actual hard frozen nucleus of the comet is somewhere in the middle of the white area. Most of what we see in the picture is gas expelled from the comet. The nucleus of the comet is probably smaller than the earth. Since it doesn't have enough gravity to hold these gases next to its surface, it's probably smaller than 500 miles in diameter. This of course is assuming it's a mostly normal 'comet.'

There were two excellent chances for astronomers to figure out the actual size of Hale-Bopp's solid nucleus. In May of 1996, the comet passed behind the moon. In early October of 1996, it passed in front of a star. Both of these alignments would have given astronomers a chance to find out where the bright gases of the comet ended and the hard surface of the comet began. In both cases I have found a string of excuses why these measurements were not done (or not released).

NASA said early on that the comet "could be as small as 25 miles" and still produce this bright huge cloud of gases. They seemed to want to lowball the comet size. The Hubble has probably figured out by now exactly how big the comet is, but they are not releasing the pictures. The most recent public pictures of the comet are a year old when it was much farther out.