how small does a hole have to be to completely keep water out but let air in?

GOOD Q

depended on pressure in the water

cell "skin" passes vapor, gas atoms not liquids

as a rough estimate the size of atoms about Angstrom units >> 1/ 10,000 millimeter

BUT water has surface tension so at tiny sizes the "strength of the surface is high

mm is too big. micron 1/1,000 maybe

there are fabrics that "breathe' but are water resistant

Would be nice to know your context. A tent that is "waterproof" will let water through if it is touched and the surface tension released.

A small hole will seal over with water surface tension, but be pretty useless in meeting the needs of a person and if the water is applied (like rain) that is completely different from submerging a vessel with one or more small holes. Applied water has no pressure although a large enough hole will allow capillary action. which for certain sizes is quite strong http://en.wikipedia.org/wiki/Capillary_action

And, thinking about it - the water molecule is smaller than the Oxygen and Nitrogen molecules so in the ultimate, no "hole" will keep out water and admit air.

air is made up of a few elements, some of which individually are larger than the water molecule.