Microsoft put together some UI guidelines for designing their Windows 8 store apps. These could apply to any touch based UI.
Take a look at the section Touch, Commanding, and Controls beginning on page 65
Windows 8 User Experience Guidelines:
Direct download: http://go.microsoft.com/fwlink/p/?linkid=258743
In particular relating to minimum sizes for touch see page 74-75
People often blame themselves for having “fat fingers.” But even baby
fingers are wider than most touch targets. The image on the left shows
that the width of the average adult finger is about 11 millimeters
(mm) wide, while a baby’s is 8 mm, and some basketball players have
fingers wider than 19 mm!
7×7 mm: Recommended minimum size 7×7 mm is a good minimum size if
touching the wrong target can be corrected in one or two gestures or
within five seconds. Padding between targets is just as important as
When accuracy matters Close, delete, and other actions
with severe consequences can’t afford accidental taps. Use 9×9 mm
targets if touching the wrong target requires more than two gestures,
five seconds, or a major context change to correct.
When it just
doesn’t fit If you find yourself cramming things to fit, it’s okay to
use 5×5 mm targets as long as touching the wrong target can be
corrected with one gesture. Using 2 mm of padding between targets is
extremely important in this case.