You might just have to try it on a monitor and measure it. I mean, if I took a certain graph and showed it on my laptop and simultaneously on a large monitor in a conference room, they would each be a different size in cm, right? So how is your code supposed to know?
You might look up "DPI-Aware Behavior in MATLAB" in the help:
"Root ScreenSize Property
The ScreenSize property of the root object might not match the display size reported by high-DPI Windows systems. Specifically, the values do not match when the Units property of the root object is set to 'pixels'. MATLAB reports the value of the ScreenSize property based on device-independent pixels, not the size of the actual pixels on the screen."
You might check out groot:
Graphics Root with properties:
CurrentFigure: [0×0 GraphicsPlaceholder]
ScreenSize: [1 1 1920 1080]
MonitorPositions: [1 1 1920 1080]