MongoDB documents are currently restricted to a maximum of 16MB, so assuming Gilbert's calculations are accurate, you wouldn't be able to store all 6 Million user_ids in the Post document.
However, you could consider storing the votes in the User document instead (i.e. the post_ids that the particular user voted for). It is much less likely that a user votes on 6 million different posts, so this way you won't reach the size limit as quickly.
Another way to handle this: If you expect that many votes for a particular post, you may want to store the votes outside the Post documents in a separate collection and do an additional query, similar to a many-to-many JOIN table in SQL fashion:
user_votes { user_id: ObjectId(...), post_id: ObjectId(...), vote:-1 }
and create a compound index on (user_id, post_id).