An iframe is a possibility; the fixed box size is an user experience tradeoff. I really believe in the vision of a read-write web, i.e. for me the default should be all involved parties being able to comment on any given content.

Of course this should be possible to turn off for certain content and moderated, but it still seems artificial to not have the discussion on the same page, compared to the original post. HN is beautiful, for instance. :)