Any square unit you like can be used to measure the area of a postcard, however to make the numbers have a reasonable size and ease of handling, I would suggest using square centimetres as a postcard is about hand size (and likely to be about 15cm by 10cm).
cm's centimeters
grams
metric unit
miles?
Millimeters
The metric unit of length is the 'meter'.
Grams
Millimeters
Square centimetres (cm2).
metric unit
kilograms
A microgram.
meters
miles?
Litres.
Tonnes
Millimeters
The metric unit of length is the 'meter'.
You would use meters.