Trowel size

What notch sized trowel should be used for 16x16 tile on concrete slab

At least a 3/8th, but in most cases I would use a 1/2 inch notch.

Use a 1/2" notch to spread thinset on the substrate AND backbutter the tiles with the smooth side of the trowel before placing them in the combed thinset.

Comb the thinset on the substrate in one direction only, usually left to right or right to left in front of you. This way after backbuttering the tiles you can easily place the tile in front of you and cause the ridges to collapse uniformly into the grooves. This will allow much better coverage than spreading the thinset willy-nilly with a lot of curves and disruptions in the spread pattern.

If you find you are getting too much thinset purging upward between the tiles then drop back to a smaller trowel using a 1/4" X 3/8" X 1/4" for the substrate and continue to backbutter the tiles with the smooth side of the trowel.

Thank you, that is the way I will do it,
This is a volunteer job for Habitat for Humanity

Sorry, One more question,
What brand/type of thin set to use. They shop at Lowes and Home DEpot

Either "versabond" from HD or "multipurpose" from Lowes. A step up would be "flexbond" or "multipurpose pro" from HD or Lowes, respectively. This slab is clean with no cracks, paint, sealers, adhesive residues, etc?

Good for you for volunteering with H4H. My dad, now retired, spends a great deal of time now working on their homes and in their store in his part of South Carolina.