How font sizes at iPhone 6/6+ compare to earlier iPhones?
Above all, text must be legible. If users can’t read the words in your app, it doesn’t matter how beautiful the typography is. When you adopt Dynamic Type in your app, you get:


Automatic adjustments to letter spacing and line height for every font size
The ability to specify different text styles for semantically distinct blocks of text, such as Body, Footnote, or Headline
Text that responds appropriately to changes the user makes to text-size settings (including accessibility text sizes)


Make sure all styles of a custom font are legible at different sizes. One way to do this is to emulate some of the ways iOS displays font styles at different text sizes. For example:


Text should never be smaller than 11 points, even when the user chooses the extra-small text size. For comparison, the body style uses a font size of 17 points at the large size, which is the default text-size setting.
In general, font size and leading values differ by one point per text-size setting. Exceptions to this are the two caption styles, which use the same font size, leading, and tracking at the extra-small, small, and medium settings.
At the smallest three text sizes, tracking values are relatively large; at the largest three text sizes, tracking values are relatively tight.
The headline and body styles use the same font size. To distinguish it from the body style, the headline style uses a heavier weight.
Text in a navigation controller uses the same font size that body style text uses for the large setting (specifically, 17 points).
Text always uses either regular or medium weight; it doesn’t use light or bold.


iOS Human Interface Guidelines: Color and Typography