In the SI - the current metric system - that would be the kilogram. In older metric systems, the gram has been used as the unit of mass.
You would measure that in grams, or milligrams.
It depends on what you mean by size: its length, volume, surface area or even its mass.
The metric unit of mass is kilogram. It is the mass of a piece of platinum alloy that keep in a museum in Paris. One kg (kilogram) is equal to 1000 g (gram). One kg (mass) weighs 2.205 pounds on the surface of the Earth.
Kilograms are the standard unit of mass in the metric system. Therefore, grams or milligrams can be used to get the mass of smaller objects.
I would use grams.
The kilogram would be logical.
The metric unit of mass is the kilogram. Why you would need to measure the mass of a necktie is beyond me.