It depends on ewhat attribute of the thumb you wish to measure:
Some attributes of possible interest are length, mass, conductivity, tensile strength.
If length, probably centimetre. Millimetre or smaller if you wanted an absurd degree of precision.
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You would use area. I would use the meter2. Okay
You would use Volume.
You would use either inches or centimeters.
I would use centimetres and millimetres
Honestly you could use any unit of measure! I would use inches. Thanks Cheezpuffz