I wouldn't be surprised if it stays at 5.5. Remember they made the Note 2 narrower so they may be reluctant to increase the width. Personally I would probably prefer 5.5 over a larger size and certainly anything above 6 would be pushing it.
Actually I don't think it would be a bad idea to have two versions of the Note 3: 5.5 and around 6-6.3. There is probably a market for the latter.