Height is the vertical distance measured straight from the bottom of something to the top.

Similarly, length is also the distance between two points measured along a straight line.

As the largest side of the box defines the side of the box which is lengthiest in all sides.

In SI units, the length is measured in meter. It is defined as the distance travelled by light in vacuum in $1/299792458sec$.

Let’s know the units of length in different systems.

There are basically four systems of units $CGS$, $FPS$, $MKS$ and $SI$ system.

In $CGS$ system the unit of length is centimeter, while in
$FPS$ system, its unit is foot.

In $MKS$ & $SI$ systems, the unit of length is meter.

$SI$ unit is called the metric unit system.

The non-metric units of the length are those units which neither use the metric unit or $SI$ unit nor the metric prefixes.

The metric or the $SI$ unit of the length is $meter$ denoted as $m$.

Hence, centimeter, nanometer or the decameter are the metric units because these are using the metric prefix

The units like angstrom, feet, inch are the non-metric units of the length because, the above units of the length are neither metric nor $SI$ nor using the metric prefixes.

Now, let’s study the smaller and bigger unit of length.

The smallest unit of length is $fermi$. It is denoted by $f$ and is equal to $10_{−15}m$.

To measure the wavelength and the diameter of an atom, Angstrom is used denoted as $A_{o}$ which is equal to the $10_{−10}m$.

To measure very large distances, the astronomical unit (AU), light year (ly) and parsec (parsec) are used.

One Astronomical unit (1 AU) is the average distance of the sun from the earth, and $1AU=1.496×10_{11}m$.

The light year is bigger than the astronomical unit. It is the distance travelled by the light in one year which is equal to $9.46×10_{15}m$.

The parsec is the biggest unit of the length or distance. It is the distance at which the average radius of earth’s orbit subtends an angle of one arc second.

The parsec is equal to the $1parsec=3.08times10_{16}m$ or $1parsec=3.26ly$.

Revision

The length is the distance between the two points measured along the straight line.

The length is measured in meter. The meter is defined as the distance travelled by the light in vacuum in $1/299792458sec$.

The non-metric units of the length are those units which neither use the metric unit nor $SI$ unit nor the metric prefixes.

The smallest unit of the length is $fermi$ denoted by $f$. It is equal to $10_{−15}m$.

The parsec is the biggest unit of the length or distance. It is the distance at which the average radius of earth’s orbit subtends an angle of one arc second.