Samsung's newest media player thingy (like the iPod is to iPhone) has a 5.8" screen. It is a very low quality screen, but the dimensions are there. At that point, I don't see why they don't make a phone in that size soon enough.
If that were the case, it would be a very small thing to hit the 6" factor.
The Note is an everyday device. I don't want them to get away from that. Tablets are a couch device: something that is very easy to carry around with you, but not something you feel that you can't leave the house without. Your primary phone, on the other hand, is an everyday device. More people are willing to leave the house without their wallet then they are to leave without their phone. This being the case, I'm certain we will find a sweet spot of largest possible device to still be an everyday device without it being relegated to a couch device. The 7" tablets I've played with are probably on the wrong side of that border. I think 7" is probably too big to be an everyday device. But it is borderline. I think if I spent more than just a couple minutes I'd find it easily incorporated into my life. But maybe not.
So, 7" is definitely the max. The maximum usable everyday device probably falls between 5.3" (original Note) and 7". I think we'll see that 5.5" (note2) is easily an everyday device. The range between 5.5 and 7 is fairly small, but those small differences will mean a lot, I think, since we are on the borderline. 6" is not out of the realm of possibility. I would say that it is not out of the realm of probabiliby either.