According to WCAG 2.0 documentation
2.2 Touch Target Size and Spacing
The high screen resolution of mobile devices means that many interactive elements can be shown together on a small screen. But these elements must be big enough and have enough distance from each other so that users can safely target them by touch.
Best practices for touch target size include the following:
Ensuring that touch targets are at least 9 mm high by 9 mm wide, independent of the screen size, device or resolution.
Ensuring that touch targets close to the minimum size are surrounded by a small amount of inactive space.
Note: Screen magnification should not need to be used to obtain this size, because magnifying the screen often introduces the need to pan horizontally as well as vertically, which can decrease usability.
In another place, it is described as no smaller than 45px wide and 45px tall as the minimum size.
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