How can i set the width of a table?

I would use the tabularx package. An argument to the tabularx environment specifies the table width, X columns share the available space.

A simple example:

\usepackage{tabularx}

\begin{tabularx}{.8\textwidth}{lXXr}
left column & text & text & right column
\end{tabularx}

See also:

• Relative column width in LaTeX

• How to force a table into page width

Use a tabular* environment, with @{\extracolsep{\fill}}, as in

\begin{tabular*}{.5\linewidth}{@{\extracolsep{\fill}}ccc}
  One& Two& Three\\
  Four& Five& Six
\end{tabular*}

(where you replace .5\linewidth with something wide enough for all of the tables).