Mass is measured in kilograms.
Mass and weight are two different things: you cannot measure the mass of your weight. That is like talking about the temperature of your height - it makes no sense.
It depends on whether you want to measure mass or weight so your question needs to be more specific.
yes
The metric unit for measuring human weight is actually the Newton (N). Newton's are a measure of force and weight is the product of the acceleration of gravity and an objects mass. The metric unit for measuring human mass is the kilogram (kg).
A gram would be an appropriate measure for its mass.
Tape measure
3 milligrams is the weight of an average ant.
Nothing, since a metric ton is a measure of mass, not of weight.
grams or meters weight or length
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).
Weights are measured in newtons. Note that the mass is measured in kilograms; in popular culture, mass and weight are often confused.
The answer will depend on what aspect of the calculator you wish to measure: its length, volume, mass, weight, temperature, etc.