Calibrate your display with a bank card, then use an on-screen centimetre ruler for quick size estimates.
Browsers do not reliably report physical pixel density, so a known-size card provides the reference needed for a closer estimate.
Estimate a small object, Check print dimensions, Compare card or label sizes, and Measure an on-screen layout
Recalibrate when you change browser zoom, display scaling or monitor because each setting changes the ruler's physical size.