Changing the font size in a table

Scale down your table to the textwidth

\documentclass{article}
\usepackage{graphicx}
\begin{document}

\begin{table}
\resizebox{\textwidth}{!}{%
  \begin{tabular}{cc}
    Knuth & Lamport
  \end{tabular}}
\end{table}

\end{document}

then you have the optimal font size. However, all tabular lines are also scaled down which doesn't matter because it looks nicer.

Write \tiny immediately after \begin{table}. If you don't use a (floating) table environment, enclose your (e.g.) tabular environment in a group and write \tiny after \begingroup.

\documentclass{article}

\begin{document}

\begin{table}
\tiny
\centering
\begin{tabular}{cc}
Knuth & Lamport
\end{tabular}
\end{table}

\end{document}

EDIT: To change the fontsize for all tables (or even floats of every type), one may use the floatrow package (this also saves typing \centering in every table):

\documentclass{article}

\usepackage{floatrow}
\DeclareFloatFont{tiny}{\tiny}% "scriptsize" is defined by floatrow, "tiny" not
\floatsetup[table]{font=tiny}

\begin{document}

\begin{table}
\begin{tabular}{cc}
Knuth & Lamport
\end{tabular}
\end{table}

\end{document}