A common house fly is a very small animal. In order to get an accurate measurement, you would have to use millimeters.
Milimeter
square foot square yard acre square meter
Centimeters
metric unit
Of course, how else would you measure how much milk you'd have? You could measure in Imperial gallons, but the metric Litre (or Liter) is also a common way to measure liquid substance (which what milk is, a liquid).
I would go with millimetres
Milimeter
Lengths are measured in meters.
square foot square yard acre square meter
Metres
A microgram.
Potatoes are sold by the kilogram.
A thermometer.
distance
Centimeters
kilograms
you would measure it in kilograms