Use dry measure measuring cup.
a millimeter
You must first decide what you want to measure about the rice grains: their length, circumference, mass, weight, volume, color, elasticity, etc.
A balance.
use scale
Units of measurement are useful to rationally measure anything.
you would use a kilogram
use a tape measure or some form of measurement to measure you ;]
inches
cm
a meter
Joules
You would use CM or INCHES to measure a knife