The actual area being measured is determined by the distance-to-spot ratio of the IR thermometer you are using, and this varies from model to model. As the distance from the object being measured increases, the spot size of the area being measured also increases.
For example, Sper Scientific’s IR Pen has a distance to spot ratio of 1:1 meaning that at 1" from the target it will measures the temperature of a 1" circle. At 2" it will measure a 2" diameter and so forth.
See Sper's full line of Infrared Thermometers here.