meters
A door is about 2 meters, but most likely you'd measure in centimeters.
While the dog is standing up on all fours, measure from the floor to the top of the shoulder.
I would use Meters
One would be a scratch test. The quartzite is much harder than the marble. Another test would be the use of dilute hydrochloric acid, which will cause decomposition of the calcite minerals forming the marble.
I would use a micrometer. A caliper would do as well.
Millimeters.
You should clarify what you want to measure about it: its diameter, mass, weight, color, etc. If you want to measure its diameter, that would usually be specified in microns (= micrometers).
That depends what aspect of a marble you want to measure, for example its diameter, its mass, its density, its color, etc.
Meter.... or more likely millimeters
Calipers would be accurate, a micrometer would probably be most accurate. there's a few options, depends on your purpose.
6.5cm
Grams
To measure a diameter, or length, you would use meters, centimeters, or millimeters. To measure the surface area, use square meters, etc. To measure the volume, use cubic meters, etc.
Centimeters would be most convenient metric unit for the diameter of a dinner plate.
The metric unit typically used to measure the thickness of guitar strings is millimeters (mm). This measurement helps determine the gauge or diameter of the string, which in turn can affect the tone and playability of the instrument.
If I was you i would use millimetres or centimetres.