As a thought experiment, packing up the whole of a website is a good upper bound. If we were doing this for real, we could further aggressively trim, taking advantage of the fact that content utility tends to follow a power law rather than a uniform distribution. For instance, take stack overflow; plink everything with a negative point score. Plink everything with no replies and no votes. Plink everything that has a reply with more than, say, 5x the points (i.e., if there's a 100 point reply and a 4 point reply, plink the four-point reply). Plink entire sites/tags that are unlikely to be useful (no Python software on the mission? No Python interpreter shipped? Remove all Python questions). For Wikipedia, remove all information about localities that have either less than 5KB of text or 10,000 people. Spend some time hitting the "random" button on Wikipedia and you can get a sense of how much of Wikipedia does not need to be sent into space. Remove vast swathes of the catalog of each species on Earth... I doubt a Martian settler is going to have a compelling reason to suddenly look up a cricket particular to northern Utah. And so on.

You could probably cut all those estimates down by 90%, with very little loss observable to the users. It wouldn't be zero loss, but use the savings to stuff other useful stuff in there instead, such as redundant copies to ensure we don't lose anything.