The exact value of all 401k isn't really known, but the average account value is estimated at ~135k, if (let's say) 200 million Americans have a 401k, that comes out to 27 trillion.
It's mostly known, but you're probably right it's not exactly known
45 trillion across all retirement accounts, 12 trillion in defined contribution (including 401k), 9 trillion in just 401k
https://www.ici.org/statistical-report/ret_25_q2
In that chart, the 18T of IRA has portion in private equity, and the 15T of pension (defined benefit) has a portion in private equity. The 13T defined contribution plans (of which the vast majority is 401k) can't be placed in private equity right now.
The economic vampires must salivate about this opportunity all night and day.