According to some iOS 8 files found inside Xcode 6 SDK, the resolution of the iPhone 6 display will be 828 x 1472.
Although the number looks like someone pulled it out their backside, there seems to be some logic behind it. One of the files within the iOS 8 Springboard was optimized for a display resolution of 414 x 736. But since the SDK parses hardware resolution via point values, the actual resolution is usually double (or possibly triple) of that number. Hence, 828 x 1472.
828 x 1472 at 4.7-inch works out to be around 360PPI, which is more than the current iPhone display and 'Retina' enough. It would be a stretch at the purported 5.5-inch size, which is where Apple might go for 3x the 414 x 736 value, and reach 1242 x 2208, although we are now far too deep into speculation territory.
Whatever it is, we have about three weeks left now to find out for...