\documentclass{article}
\setlength{\textwidth}{14cm}
\newenvironment{newidth}[2]{
 \begin{list}{}{
  \setlength{\topsep}{0pt}
  \setlength{\leftmargin}{#1}
  \setlength{\rightmargin}{#2}
  \setlength{\listparindent}{\parindent}
  \setlength{\itemindent}{\parindent}
  \setlength{\parsep}{\parskip}
 }
\item[]}{\end{list}}

\begin{document}
textwidth 14cm. textwidth 14cm. textwidth 14cm. textwidth 14cm.
textwidth 14cm. textwidth 14cm. textwidth 14cm. textwidth 14cm.
textwidth 14cm. textwidth 14cm. textwidth 14cm. textwidth 14cm.

\begin{newidth}{3cm}{3cm}
left and right margins are increased by 3 cm. left and right
margins are increased by 3 cm. left and right margins are
increased by 3 cm. left and right margins are increased by 3 cm.
left and right margins are increased by 3 cm. left and right
margins are increased by 3 cm.

   \begin{newidth}{3cm}{-1cm} %newidth 안에 newidth 가능 (nesting)
    The left margin is increased by 6cm and the right margin is increased by 2
    cm. The left margin is increased by 6cm and the right margin is increased by 2
    cm. The left margin is increased by 6cm and the right margin is increased by 2
    cm. The left margin is increased by 6cm and the right margin is increased by 2 cm.
  \end{newidth}
\end{newidth}
textwidth 14cm. textwidth 14cm. textwidth 14cm. textwidth 14cm.
textwidth 14cm. textwidth 14cm. textwidth 14cm. textwidth 14cm.
textwidth 14cm. textwidth 14cm. textwidth 14cm. textwidth 14cm.

\begin{newidth}{-0.5cm}{-0.5cm}
textwidth 15cm. textwidth 15cm. textwidth 15cm. textwidth 15cm.
textwidth 15cm. textwidth 15cm. textwidth 15cm. textwidth 15cm.
textwidth 15cm. textwidth 15cm. textwidth 15cm. textwidth 15cm.

\end{newidth}

\end{document}