\documentclass[12pt]{book}
%
\makeatletter
\def\table{\@ifnextchar[{\table@i}{\table@i[\fps@table]}}
\def\table@i[#1]{\@float{table}[#1]\tiny}
\makeatother
%
\begin{document}
%
normal text size

\begin{table}[b]% special placement is possible
\centering
\begin{tabular}{l}
abc\\
ABC\\
$abcABC\alpha\Gamma\int$
\end{tabular}
\caption{ein kleines table}
\end{table}

normal text size

\end{document}