Apple starts with the 57px icon and a radius of 10 then scales up or down from there. Thus you can calculate the radius for any icon size using 10/57 x new size (for example 10/57 × 114 gives 20, which is the proper radius for a 114px icon)

The comfortable minimum size of tappable UI elements is 44 x 44 points.

Top picks so far in terms of a) tip sensitivity and b) feel in your hand: AluPen, Cosmonaut, Adonit Jot Pro

The iPad’s split keyboard has phantom buttons. If you’re used to typing Y with your left hand, you can.

Snapshot of 500 Android screen sizes on EU site...

...The first culprits are embedded web views—browser views embedded within apps such as Twitter or Facebook, enabling users to consume links and content without ever leaving the app itself. These views often incorporate their own chrome which results in slightly smaller (or at the very least different) dimensions than the native browser...

...But this is only the half of it. On Android specifically, a series of personalisation and accessibility settings further contribute to screen size diversity. The most disruptive setting by far is Zoom Level.

iWebInspector is a free tool to debug, profile and inspect web applications running on iOS Simulator