At the time Comet ISON (C/2012 S1) was discovered back in September 2012, it was calculated to be a staggering 584 million miles from the Sun. Despite this distance being somewhat incomprehensible, compare this to Comet Hale-Bopp’s discovery back in 1995 when it was at an even more incredible distance of 665 million miles. From these two distances, and of course the advances in optical technology over the last 17 years, I’d automatically assume that ISON was actually a little smaller than Hale-Bopp – otherwise, might we have expected to discover ISON a little earlier in 2012?
http://www.cometison2013.co.uk/