The density of air is about 1.2 kilograms per cubic meter (water is 1000 kg/m3). So, it'd take about 830 cubic meters of air to make a ton (1 m x 1 m x 830), or a cube with each side of length 9.4 meters or about 30 feet.

A ton is a ton, regardless of phase (solid, liquid, gas, feathers)