Video 170
In this video I demonstrate how to create and use Attribute Expressions.
To return the area of a polygon use the following in your expression builder:

Floor(Area($feature,'square_kilometers'),2) + "sq km"

Other units available are:

acres | squarefeet | hectares | squarekilometers | squaremiles | squarenauticalmiles | squaremeters | squareyards

Attribute Expressions are a good way to add further intelligence to your popups without having to add more fields to the underlying feature service.