Evidence has been found of giant squid over 75 feet in length. However, it would appear that this Colossal Squid has shorter limbs, but a larger body, making it heavier. And I believe the article does mention that this specimen is "half grown".
As for giant squid attacking sperm whales, this is now almost universally accepted to be incorrect. It was a story that grew out of the evidence of succer-disk scars on the hides of sperm whales. But it is now generally accepted that these scars are the result of the whales attacking the squid, which appear to constitute a chief food source for them.