Square centimeters
You would probably measure a keyboard in centimeters (cm).
To do it literally you could use cubic inches or cc, but wouldn't word count be more useful in comparing books with each other? Or even page count?
Either of the first two if you wish to measure its capacity; the latter if you want to measure its length or thickness.
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.
Centimeters.
square centimeters
square centimeters
A tape measure that measure in feet and inches (or meters and centimeters) would be most appropriate.
Usually meters would be an appropriate unit.
What is the most appropriate instrument to measure
Cubic centimeters would be most appropriate for volume
You would measure it in centimeters.
20 cm times 15 cm = 300 square cm
I would go with inches but you could measure it in centimeters.
centimeters
centimeters