According to the Japanese blog MacOtakara [Google Translate] Monday morning, Apple will use 'iPhone 6' and 'iPhone 6 Plus' monikers with respect to a 4.7 and 5.5-inch edition of the upgraded smartphone.
The French blog NowhereElse.fr said last week that the bigger iPhone 6 model will be marketed under the 'iPhone Air' moniker. Unlike 'Air,' Apple's never used the 'Plus' suffix to market an iPhone, or any other product for that matter. If true, this'll be a first.