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.
what unit would you the length of a shoe? mm,cm,dm,m, or km.
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