what would the best pneumatic gun be to face nail wood flooring?
what size nails would be used?

Depends on the thickness of the wood. For 3/4" solid, I prefer 2" 15 gauge Senco nails. They have a slightly larger finish head that holds better, IMO. For 1/2" solid, a 1&1/2" long nail but same gauge and brand. For thinner solids, you can use brad nails, 18 gauge, 1" to 1&1/4". For engineered, brads are fine until you get up to 9/16" or thicker, then I go back to 15 gauge. The idea is, the thicker the wood, the thicker the nail. Some may disagree, that's cool. This is the way I like to do it and have always done it. I like Senco but I hear Hitachi is good as well. Many really liked the new Bostitch N62 but now some are reporting problems.

thanks.