Either of the first two if you wish to measure its capacity; the latter if you want to measure its length or thickness.
You would probably measure a keyboard in centimeters (cm).
Square centimeters
A tape measure would be the tool, measuring in centimetres or inches.
To measure a thumbnail the best unit of measurement would be inches or centimeters. A standard rules in the United States has both measurements on it, and the thumbnail can easily be measured.
In the metric system, centimeters. In the English system, inches.
Milliliters
You would measure it in centimeters.
I would go with inches but you could measure it in centimeters.
centimeters
centimeters
centimeters.
centimeters
With a tape measure.
You would probably measure a keyboard in centimeters (cm).
you would measure a wall in meters
Centimeters
you measure it i centimeters