How to control enumitem's description list via leftmargin and labelwidth keys

With version 3.3 it's:

\begin{description}[
  leftmargin=!,     % let enumitem do the dirty job
  labelwidth=\widthof{\bfseries The Largest Named Item}]

You have to put the format of the label (here, something similar to widest would be useful, so I've added it to my todo list)

\newenvironment{mydesc}[1]
  {\settowidth{\dimen0}{#1}%
   \renewcommand{\descriptionlabel}[1]{##1\hfill}%
   \begin{description}[leftmargin=\dimexpr\dimen0+\labelsep\relax,labelwidth=\dimen0 ]}
  {\end{description}}

\begin{mydesc}{The Largest Named Item}
\item[Small Name]
    \ShortDescription
\item[The Largest Named Item] \LongDescription
\item [Larger Name]
    \ShortDescription
\end{mydesc}

If enumitem version 3 is not available, it's possible to use a solution provided in the LaTeX Companion:

\newenvironment{mydesc}[1]
  {\list{}{\renewcommand\makelabel[1]{##1\hfil}%
     \settowidth\labelwidth{\makelabel{#1}}%
     \setlength\leftmargin{\dimexpr\labelwidth+\labelsep\relax}}}
  {\endlist}