Square meters, generally.
metres
meters
Litres.
Any metric unit would do. But it might be easier to pick something smaller than the length of the pencil, like a centimeter, or a millimeter.
Square meters.
Square metres
meters2
A square nanometer.
Square metre
To find the area of a book cover in metric units, the most appropriate unit to use would be centimeters. The formula for finding the area of a book cover would be: length x width.
You would use square metres (m2) to find the area of a door.
I would use the metric unit is inches
Square meters, generally.
Square centimetre.
Square centimetres (cm2).
Millimeters