A balance.
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use scale
a millimeter
You must first decide what you want to measure about the rice grains: their length, circumference, mass, weight, volume, color, elasticity, etc.
Personally, I would measure the length of a grain of rice in millimeters.
It is not possible to give a sensible answer to the question. A square metre is a measure of area in 2-dimensional space whereas the mass of rice depends on the volume - a 3-dimensional measure - of rice. The height of the pile of rice will affect the mass of rice.