You would find millimetres on a ruler using metric.
You would find centimeters, millimeters and meters on a metric ruler or tape measure.
That one!
One can find a metric to imperial conversion at a neighborhood library. A bookstore would be another source to find a metric to imperial conversion table.
The liter is commonly used. For a small bottle, milliliters might also be used.
Metric ruler
The SI unit would be grams, however considering a blade of grass is pretty small I would probably measure it in mg, milligrams.
You would use meters.
Grams.
a millimeter
millilitres
Millimeter