Getting small font size in all text of a table

You can use for example {\scriptsize ... } around your \begin{tabular}-\end{tabular}. this will shrink everything to the given size.

for example:

\documentclass{article}
\usepackage{booktabs}
\begin{document}
\begin{table}[ht]
 \caption{Meteorological instruments.}
  \centering\scriptsize
   \begin{tabular}{l l }
\toprule
Measured parameter  &Sensor make and model [serial \#] \\
\midrule
Temperature         &Pt100 in Stevenson screen \\
Barometric pressure &Setra 278 (CS100) [3141007]\\
\bottomrule
   \end{tabular}
 \label{tab:sensors}
\end{table}
\end{document}

This example uses the booktabs package for the rules.

text

{\small% it is a switch not an environment
... your tabular
}

text