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

The hole would have to be smaller than a water molecule yet bigger than an "air" molecule (we will assume O2, however, the "air" is made up of 78% nitrogen.) Yet, an O2 molecule is larger than a water molecule, so this is not possible. You see, the hole would have to large enough to fit the O2 molecule, which the H20 molecules would easily side in the hole at a greater velocity.