People interact with touch-based user interfaces with their fingers. So user interface controls have to be big enough to capture fingertip actions without frustrating users with erroneous actions and tiny targets. Ok, so how big? — by Luke Wroblewski
In the iPhone Human Interface Guidelines, Apple recommends a minimum target size of 44 pixels wide 44 pixels tall. Since physical pixel size can vary by screen density, Apple’s pixel specifications apply best to the iPhone’s 320 by 480 pixel, 3.5 inch display (164ppi). Since the release of the iPhone 4’s Retina Display (326ppi) Apple has updated these specs to points instead of pixels.
In the Windows Phone UI Design and Interaction Guide (PDF), Microsoft goes further and suggests: a recommended touch target size of 9mm/34px; a minimum touch target size of 7mm/26px; a minimum spacing between elements of 2mm/8px; and the visual size of a UI control to be 60-100% of the touch target size.