It should be before October, because the filings say they will run 10.13, and October would quite probably imply 10.14.
As for a 15’’ MacBook, I’m also hoping for it. Yesterday I went to an Apple Store to repeat the “test” I did last year (holding a 15’’ MBP with one hand), and, I could bear with it if there are no lighter options, but I prefer it lighter).
The fact that the new MBP have substantially increased specs (cores and RAM) that push them more into the Pro field, helps keep my hopes high for a larger screen Macbook coming, as this MBP bump could be a resource for differentiating the MBP from MacBooks with lower specs but with larger than 12’’ display.
I really hope and wish all of this will happen before October, though.