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).
You would use the metric unit "liters" to measure the capacity of a hot water tank.
You would typically use the metric unit of centimeters to measure the growth of your hair.
You would use the metric unit "kilograms" to measure the weight of a bag of groceries.
You would use the metric unit "kilograms" to measure the weight of a sack of peaches.
The metric unit used to measure a picnic cooler would be liters (L) for volume.
Millimeters
Square centimetres (cm2).
You would use the metric unit "liters" to measure the capacity of a hot water tank.
You would typically use the metric unit of centimeters to measure the growth of your hair.
You would use the metric unit "kilograms" to measure the weight of a bag of groceries.
kilograms
You would use the metric unit "kilograms" to measure the weight of a sack of peaches.
You would use the metric unit "kilograms" to measure the mass of a wardrobe.
The metric unit used to measure a picnic cooler would be liters (L) for volume.
The metric unit commonly used to measure height is the meter. So, you would measure the height of Big Ben in meters.
meters
Litres.