The “spot size” of any given measurement is controlled by two variables:

The distance-to-target ratio (or spot ratio) of your particular infrared thermometer
The distance between your infrared thermometer and the target
Typically listed on the thermometer itself, the distance-to-target ratio (DTR) – or spot ratio – tells you the diameter of the circle of surface area an IR thermometer will measure at a given distance.

For example, an infrared thermometer with a 12:1 DTR ratio will measure the temperature of a 1-inch diameter circle of a surface area from 12 inches away.