用p或者m可以控制每列的宽度（需载入array宏包）。
要整个表格相对于页面居中，用chngpage宏包的adjustwidth,如下：
\documentclass[oneside]{article}
\usepackage{chngpage}
\usepackage{array}
\begin{document}
\begin{adjustwidth}{-1cm}{-1cm}
\begin{tabular}{|p{.2\textwidth}| p{.2\textwidth}| m{.7\textwidth}|}
\hline
new & old & all boring, all boring, all boring, all boring, all boring, all boring, all boring, all boring, all boring, all boring, all boring, all boring, all boring, all boring, \\ \hline
\end{tabular}
\end{adjustwidth}
\end{document}