http://www.fda.gov/downloads/drugs/guidancecomplianceregulatoryinformation/guidances/ucm240243.pdf

in this guidance the size of sprinkle granules, coated beads etc was proposed to be

0.82 to 3.04 mm before swallowing and the average value will be 1.93 i.e 2 if you round it .so 2mm 10# is the regular size for a particle to get dissolved as per US.

In USP, 10 # size is to pass all type pf granules or particle even big size as of 1.5 to 2 mm size  like sprinkle granules and coated beads etc. Disintegrating apparatus is having some big size mesh to avoid blockage too because it does not effect on DT time.