Change font inside document to specific point size
Just tell LaTeX to use nine point type.
\documentclass[12pt]{article}
\usepackage{setspace,lipsum}
\doublespacing
\begin{document}
\lipsum[2]
\begin{center}\setstretch{1}\fontsize{9}{11}\selectfont
\begin{tabular}{*3{l}} % <- this table should be set in 9pt
Word & Word & Word\\
Word & Word & Word\\
Word & Word & Word\\
\end{tabular}
\end{center}
\lipsum[2]
\end{document}
For a really awful result, here's what you want:
\documentclass[12pt]{article}
\usepackage{setspace,lipsum}
\doublespacing
\begin{document}
\lipsum[2]
{\strut\fontsize{9}{11}\selectfont
\begin{tabular}[t]{*3{l}} % <- this table should be set in 9pt
Word & Word & Word\\
Word & Word & Word\\
Word & Word & Word\\
\end{tabular}}
\strut\lipsum[2]
\end{document}
The fact that in a table
environment the interline spacing is reset to single spacing is a precise choice made by the setspace
package. You can revert it by removing the setting the package does.
\documentclass[12pt]{article}
\usepackage{setspace,lipsum}
\usepackage[justification=centering, font=normalsize, labelfont=bf]{caption}
\makeatletter
\let\@xfloat\latex@xfloat % remove the redefinition made by setspace
\makeatother
\doublespacing
\begin{document}
\lipsum[2]
\begin{table}
\centering\fontsize{9}{11}\selectfont
\begin{tabular}{*3{l}}
Word & Word & Word\\
Word & Word & Word\\
Word & Word & Word\\
\end{tabular}
\caption{My table}
\end{table}
\lipsum[2]
\end{document}