Hello - please see
https://docs.mcneel.com/rhino/6/help/en-us/index.htm#commands/zoom.htm
-Pascal
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Okay, but how is Zoom1To1Calibrate used?
Hello - the idea is to make Zoom1To1 show, in parallel views the true 3d size of the model on screen according to the model units. So, calibrating tells Rhino what the pixel size is and allows the scale to be correct. It is very imprecise in my opinion, to measure by hand on the display but that is how it works. You can be more exact by finding the dot pitch of your monitor (https://www.pxcalc.com/) and calculating the length of the bar from that- the bar is 718 pixels on my 4k monitor and the dot pitch is .1557 mm so I enter 718*.1557 = 111.8 mm as the length of the bar. Now Zoom > 1to1 is reasonably accurate for that monitor.
-Pascal
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Okay, I have a pitch of 0.2269mm. But where’s 718px, this is what? Height of the screen can?
Ok, it’s 478px 
So 478 * 0.2269 = 108.4582. And the default is 127. Many thanks!