\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 ΎΘΏ‘ newidth °‘΄Ι (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}