This rumor, as posted on a few Apple rumor sites, is pure detective work.
Yesterday Apple released this image - teasing for an October 14 2008 event to obviously announce new laptops.
It's hard to tell from the gloomy marketing shot, but the Apple logo is about 1.5 inches wide on my MacBook Pro. From that, this machine would appear to be under 13 inches wide, which would suggest a 13.3" diagonal. The edges, however don't look at all like the MacBook Air.