For those of you who are as curious about this as I am, I just got off the phone with one of the Marantz tech engineers. We had been emailing back and forth a few times over the past month or so, and I had sent him an email about file sizes and autosplit features, because I haven't seen anything about this possible issue yet. He called me about 10 minutes after I wrote the email, very nice guy.
Anyway, the 671 will default to autosplit at 4 gigs. (all seamless, of course). Alternatively, you can set it up to autosplit after a given time limit. So you can't tell it to autosplit after 2 gigs, but you can tell it to autosplit after 2 hours, or 1.5 hours, or whatever. In my mind, that works just as well
This thing looks pretty good, I'll probably buy in the near future.