I would recommend looking into the standards and process that Esri has provided in terms of it's measure tool
The Measure tool lets you measure lines and areas on the map. You can use this tool to draw a line or polygon on the map and get its length or area, or you can click directly on a feature and get measurement information.
You can adjust the method of measurement to take in account of Planar or Geodesic
When measuring in a data frame with a projected coordinate system, the default measurement type will be Planar. This means that 2D Cartesian mathematics are used to calculate lengths. Planar measurements reflect the projection of geographic data onto the 2D surface (in other words, they will not take into account the curvature of the earth). Geodesic, Loxodrome, and Great Elliptic measurement types may be chosen as an alternative if desired.
As this response to the post points out How accurate is ESRI world imagery basemap in ArcGIS?.
It depends where you are in the world. The World Imagery basemap is assembled from imagery from many different sources, shot at different times and with different post-processing. I don't think there is a way to find out exactly how far off it is without comparing it against some other data which has a known accuracy.
If you use the Identify tool on the World Imagery layer, it will tell you a bit more about the imagery you're seeing. Sometimes you can use that info to track down more metadata about the imagery, including accuracy information: