For an ant farm, I would suggest a kilogram. For an individual ant, a milligram.
You would measure that in grams, or milligrams.
gram
The answer depends on what characteristic you wish to measure: its mass, length, volume etc.
It depends on what you mean by size: its length, volume, surface area or even its mass.
You should clarify what you want to measure about it: its diameter, mass, weight, color, etc. If you want to measure its diameter, that would usually be specified in microns (= micrometers).
GRAMS
Tonnes
Mass is measured using kilograms.
Mass is measured in kilograms.
Mass is measured in kilograms.
The tonne.
Kilograms
Kilograms.
Grams
Probably in grams.
Kilograms
You would measure that in grams, or milligrams.