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If my understanding is correct, Chury, the best explored comet so far is rather on the smallish side as comets go. Most comets observable with naked eye were so because they flew close to Earth, not because they were so big. I wonder, though, how big are the biggest ones.

Let's get it in two variants:

  • How big is the biggest known comet? (mass, diameter)
  • Is there an estimate, or a theoretical limit on how big a comet can be? (for whatever reason, e.g. becoming a planet, breaking up due to tidal forces or whatever)
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I suppose size limit of the comet is still under process but astronomers have found a few large ones though!
The comet Holmes which appeared in 2007 became brighter and also swelled up in size. Its nuclear size is only 3.6km (2.2 miles), but it released so much dust and gas so that it expanded to about 869,900 miles (1.4 million kilometers) i.e larger than the sun.
The largest comet in terms of nuclear size was discovered in the year 1729, comet Sarabat which had a nucleus of about $100 {\rm km}$.

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A bit late for an answer, but maybe still interesting: National Geographic called comet McNaught the biggest comet measured:

In the case of comet McNaught, Ulysses found that the plasma tail slowed the solar wind inside the bow wave to half its normal speed.

The solar wind usually travels at about 435 miles (700 kilometers) a second at that distance from the sun. But inside the comet's tail, the wind was moving at less than 250 miles (400 kilometers) a second.

It took Ulysses a record 18 days to pass through the region of solar wind "shocked" by the comet's tail.

By contrast, in 1996 Ulysses had zipped through the wake of comet Hyakutake in just 2.5 days. Another ESA spacecraft, Giotto, had crossed the tail of Halley's comet in just a few hours.

The diameter of the tail of McNaught (at least on the flight path of Ulysses) could be estimated if you know the approximate speed of Ulysses. Thankfully, the mentioned Wikipedia article also says:

On May 1, 1996, the spacecraft unexpectedly crossed the ion tail of Comet Hyakutake (C/1996 B2), revealing the tail to be at least 3.8 AU in length.

With this in hand, we could conclude that the biggest measured comet tail so far is of the order of 27.36 AU in diameter.

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