How many?

Hands

The hand is a unit of measurement of length standardized to 4 in (101.6 mm). It is used to measure the height of horses in many English-speaking countries, including Australia, Canada, the Republic of Ireland, the United Kingdom, and the United States. It was originally based on the breadth of a human hand. The average shire horse is 16 to 17 hands tall and can even reach up to 18 hands.

Micrometres

The micrometre or micrometer, also commonly known as a micron, is an SI derived unit of length equalling 1×10−6 metre; that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.000039 inch). The micrometre is often used to measure the thickness or diameter of microscopic objects, such as microorganisms and colloidal particles.