How to adjust column width whilst maintaining centered text for table

Assuming you have a reasonably recent array package, then you can set the width using w, replace c by wc{2cm}

Another way that maintain the decimal align:

\documentclass{article}
\usepackage{booktabs,siunitx}   
\begin{document}
\begin{tabular}{S[table-format=8.8]S[table-format=9.7]}
\toprule
{Silicas}     & {\parbox{9em}{\centering Specific surface area\\ S [\si{\m \tothe{2} \per \gram}]}} \\ \midrule
{CARiACT Q3}  & 438.0                                                   \\
{CARiACT Q6}  & 359.0                                                   \\
{CARiACT Q10} & 303.0                                                   \\
{CARiACT Q15} & 196.0                                                   \\
{CARiACT Q30} & 112.0                                                   \\
{CARiACT Q50} & 71.0                                                    \\ \bottomrule
\end{tabular}%
\end{document}