Not sure what you mean with "light build". The SI (metric) unit for mass is the kilogram. For small objects, smaller units such as grams and milligrams can be used; for larger objects, the ton (= 1000 kilograms) is also used.
Metres
Potatoes are sold by the kilogram.
To measure the mm of an item you would need to use a metric ruler. The metric ruler has cm and mm on it.
You would measure that in grams, or milligrams.
centimeter
Meters
As a potato chip is light weight I would use grams and milligrams.
Metres
A microgram.
Potatoes are sold by the kilogram.
A thermometer.
distance
Centimeters
kilograms
you would measure it in kilograms
metric unit
You would use a ruler and measure in millimeters or centimeters.