How to control font sizes in pgf/tikz graphics in latex?

I found the better control would be using scalefnt package:

\usepackage{scalefnt}
...
{\scalefont{0.5}
\begin{tikzpicture}
...
\end{tikzpicture}
}

\begin{tikzpicture}

    \tikzstyle{every node}=[font=\small]

\end{tikzpicture}

will give you font size control on every node.