According to the FAA's AC 00-45G section 3.1, a METAR is defined:
Aviation Routine Weather Report (METAR) is the primary observation code used in the U. S. to satisfy World Meteorological Organization (WMO) and International Civil Aviation Organization (ICAO) requirements for reporting surface meteorological data. A METAR report includes the airport identifier, time of observation, wind, visibility, runway visual range, present weather phenomena, sky conditions, temperature, dew point, and altimeter setting. Excluding the airport identifier and the time of observation, this information is collectively referred to as “the body of the report.” As an addition, coded and/or plain language information elaborating on data in “the body of the report” may be appended to the end of the METAR in a section coded as
“Remarks.” The contents of the “Remarks” section vary with the type of reporting station. The METAR may be abridged at some designated stations only including a few of the mentioned elements.
What doesn't appear to be defined is exactly what area the METAR covers. It's understood that a METAR is a reporting for a particular station's current conditions at a particular time, but is there a more discrete definition for where the METAR's conditions can be expected?
The definition of a TAF specifically states that the forecast conditions are expected in an area around the reporting station with a radius of 5 statute miles. The METAR has no such specification that I can find.