The Sages taught in a baraita: If one witness says that he saw the moon two plow handles high above the horizon, and the other one says it was three plow handles high, their testimony is valid, as a small discrepancy of this kind is reasonable
How are they measuring Moon's elevation over the horizon in units of length (rather than angle)?
It is impossible to measure elevation (altitude) of a planet over horizon in meters (feet, yards &c), just like it is impossible to measure time in kilograms.
To measure elevation (which is the angle between the horizontal direction and the direction to the celestial body in the vertical plane, one needs to measure two lengths: vertical and horizontal.
E.g., the observer has to say that the Moon's elevation was 2 meters high if the measuring stick is located 4 meters away from the observer, in which case the tangent of the elevation is 0.5(=2/4) and the elevation is ~0.46 radians or ~27 degrees. If the other witness claims that the elevation was 5 plow handles when the measuring stick was located 10 plow handles away, he has said the exact same thing (elevation ~27 degrees). If the 3rd witness reports that the elevation was 2 meters (same as 1st!) when the ruler was located 2 meters away (half of what the 1st used!), then he is saying that the elevation was 45 degrees which is significantly different from what the 1st observer says.
If, instead of the plow handles, the Baraisah mentioned fingers, it would have made sense because the assumption would have been that the distance to the measurement device (palm with fingers) is the length of the observer's arm, and the ratio of finger size to the arm's length is sufficiently close for most people to be within the general measuring error for a common person (although, let it be made clear that, 500 years before Mishnah, professional Babylonian astronomers made observations with precision of minutes).