Air as such is a mixture of gases and thus does not have a defined size or cross section. However, it may be possible to define a cross section of air via the cross section of Earth's atmosphere from a point of view in outer space; this would likely not be well-defined though since Earth's atmosphere does not have a clear boundary. Of course, when talking about air in a specific region of space or in a container a cross section can be easily calculated.

In a more general sense a cross section can also refer to the cross section of individual molecules, atoms, or particles. For example, for each molecule present in Earth's atmosphere (nitrogen, oxygen, etc.) a cross section can be determined. The cross section of air could then be defined as the average cross section of the molecules, weighted by the percentage of those molecules in Earth's atmosphere. Note that in the realm of quantum physics a cross section is not defined by size but rather by the probability of an interaction occurring (in the case of air whether two molecules exchange energy and momentum by "bouncing off of each other").
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