I'd leave a public message, there is no reason for leaving a private message since my grand^10sons/daughters would only be interested in a more detailed explanation of my person. Therefore, 10 kb are not sufficient.
What message would be so important that everyone (ie. mankind) should know about in a far far future from now?
Even without knowing the exact future setting I think very little would make actual sense.
What's bad and is likely to never change and hence would be worth mentioning?