Gruber says he learned this information from people he spoke with at Apple's event on Sept. 9. Here's what he wrote:
Apple's industrial design team mocked up prototypes of every single size between 4.0 and 6.0 inches, in tenths-of-an-inch increments, and from those 20 sizes selected the two that best hit the sweet spots for "regular iPhones" and "ginormous iPhone." We might never see new iPhone sizes again - or at least not bigger ones.
If true, this wouldn't necessarily be surprising. Long before Apple unveiled the iPhone 6, reports suggested that Apple was experimenting with larger screens for its iPhone. This hints that Apple probably played with different screen sizes rather than one or two particular sizes. For example, last June Reuters said Apple was "looking at bigger screens," including one variant that measured 5.7 inches.
Regardless, it's still interesting to think that there's a giant 6-inch iPhone somewhere out there.