Question: Describe why a unit such as millimeters, centimeters or meters would be inadequate for measuring the physical extent of aspects of the visual image?

More Info to consider when answering:

How can one describe visual space in a consistent way? We could use standard units like meters. Everybody knows what a meter is and can easily measure it. But why might this not be a useful system for describing vision? Why can't we just measure how big an object is in meters and then relate that to, e.g., our thresholds or visual acuity for a certain visual pattern? Consider that our eyes capture 2D images of the world and therefore do not have access to the third dimension, and therefore we can only infer quantities such as depth.