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.