Frankly speaking I don’t know what for, but \the in front of a length command displays its value (in pt):
\the\textwidth translates to
A list of length “macros” can be found on wikibooks.
Use the layouts package to convert pt to other units including mm, cm and in:
\usepackage{layouts}
...
\printinunitsof{mm}\prntlen{\paperwidth}
In general, the layouts package enables the display of various elements of a document’s layout and experimentation with different potential layout designs.
