The iPhone 6 and iPhone 6 Plus were just announced. But as usual, phone specs never give the meta viewport device-width
sizes in CSS Pixels or DPR at widt
Below is the ios Device Sizes with detailed information
References
(css) device-width of iPhone 6 is 375px, of iPhone 6 Plus is 414px. Note that iPhone 6 Plus report window.devicePixelRatio = 3
(while not truly at 3 DPR)
Update 1: just to clarify, I believe this is NOT a prediction, but actually tested. See more detailed and reputable reference. If in doubt, you can wait for device to be out.
Update 2: for users with Display Zoom enabled on iPhone 6, css viewport in Mobile Safari fallback to 320px (like iPhone 5); for iPhone 6 Plus, it fallback to 375px (like iPhone 6).
Device-width is indeed 375x667, but that does not mean the viewport is that size. Here is an empirical sample. Note that the numbers are rounded to the 10s.
According to CSSpixels
iPhone 6: 1334x750, viewport 667x375
iPhone 6+: 2208x1242 *, viewport 736x414 *
*Virtual resolution is subsampled to fit 1920×1080 screen.