`Length[{1, 2, 3}]`

`Length`

operates on the `FullForm`

of expressions:

`Length[Exp[x]]`

`FullForm[Exp[x]]`

The length of atoms is 0:

`Length[a]`

Note that rational and complex numbers are atoms, although their `FullForm`

might suggest the opposite:

`Length[1/3]`

`FullForm[1/3]`