Which Is Bigger Air Molecule Or Water Molecule?

Air vs Water may not need to be comparable in a lab, but in practical applications while designing a hydrostatic test (tap water) versus a pneumatic test (compressor intake to the atmosphere) the size of the associated molecules and the ability to find a leak path needs to be considered.

Water will pass through a .50 opening but air will not!

I think you likely meant oxygen vs water, both molecules. O2 vs H2O. The O2 covalent bond is neutral, the H2O bonds are not, they are bipolar and the atoms closer. I would go with the O2 on that account but I should really check the electronegativity tables to be sure. At RT O2 is a gas even way cold so pretty sure O2 wins.