Question

Metric unit of a length thumb

Answer 1

The metric unit of length is the meter.

My left thumb is about 0.06 meter long.
(It's hard to tell exactly, because I'm not sure where it starts.)

Another smaller unit, called the "centimeter", is often used
for convenience.  Each centimeter is 0.01 of a meter.

So 0.06 meter, mentioned above, is the same as  6 centimeters.