Millimetres (or centimetres to one decimal place).
Inches.
the metric system
The metric unit used for measuring the width of a fingernail is usually millimeters (mm).
centimetre
Kilometres
Metres, to measure its length or width, square metres to measure its area.
I would go with millimetres
The width of a pin would typically be measured in millimeters (mm).
To measure the width of a stamp, you would typically use millimeters (mm) as the metric unit. Stamps are generally small, and millimeters provide a more precise measurement than centimeters or meters. For example, a standard postage stamp might measure around 25 mm in width.
Millimeters are commonly used. It is also possible to use cm or dm.
Because of the dimensions involved, that would be measured in km. (kilometers).
The metric unit of length is the 'meter'.