scalebox should do the trick for you. You'll need the graphicx package. For example:
\documentclass{article}
\usepackage{graphicx}
\usepackage{stackrel}
\begin{document}
\scalebox{3}{
$\stackrel[e+f+g+h^i]{a^b}{c d}$
}
\end{document}
For labels, you can use something like the following
\begin{equation}
H_{\mathrm{atom-light}}=
\underbrace{\frac{\hbar\omega_{\mathrm{a}}}{2}\sigma_3 \rule[-12pt]{0pt}{5pt}}_{\mbox{atom}}
+\underbrace{\hbar\omega\left(a^\dagger a+\frac{1}{2}\right) \rule[-12pt]{0pt}{5pt}}_{\mbox{field}}
-\underbrace{i\hbar g\left(a-a^\dagger\right)\left(\sigma^++\sigma^-\right) \rule[-12pt]{0pt}{5pt}}_{\mbox{interaction}}\,,
\end{equation}
The underbraces highlight a section, and those rule commands allow you to line them up by pushing the under braces down. Labels go in the mboxs,
The result is this
As noted by Werner, replacing \mbox with \text from the amsmath package is a little nicer because it sizes the text properly.
You may also want to consider breaking your equation over multiple lines with the amsmath package and align. This will give you more space.
\begin{align}
H_{\mathrm{atom-light}}=\ &\frac{\hbar\omega_{\mathrm{a}}}{2}\sigma_3 &&\mbox{atom}\\
&+\hbar\omega\left(a^\dagger a+\frac{1}{2}\right) &&\mbox{field}\\
&-i\hbar g\left(a-a^\dagger\right)\left(\sigma^++\sigma^-\right)\,. &&\mbox{interaction}
\end{align}
Finally, inspired by Werner I've thrown this together with TikZ:
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{equation}
H_{\mathrm{atom-light}} =
\tikz[baseline]{
\node[draw=red,rounded corners,anchor=base] (m1)
{$\displaystyle\frac{\hbar\omega_{\mathrm{a}}}{2}\sigma_3$};
\node[above of=m1] (l1) {atom};
\draw[-,red] (l1) -- (m1);
}
+
\tikz[baseline]{
\node[draw=red,rounded corners,anchor=base] (m2)
{$\displaystyle\hbar\omega\left(a^\dagger a+\frac{1}{2}\right)$};
\node[above of=m2] (l2) {field};
\draw[-,red] (l2) -- (m2);
}
-
\tikz[baseline]{
\node[draw=red,rounded corners,anchor=base] (m3)
{$\displaystyle i\hbar g\left(a-a^\dagger\right)\left(\sigma^++\sigma^-\right)$};
\node[above of=m2] (l3) {interaction};
\draw[-,red] (l3) -- (m3);
}
\end{equation}
\end{document}
which is based on an equation based on an equation... which I first borrowed from here. In your case however, perhaps it's best to build this from scratch in a TikZ picture. Remember, you can still put it into a scalebox after:
\begin{tikzpicture}[node distance=20pt]
%
\tikzstyle{boxes}=[draw=red, rounded corners]
%
\node[boxes] (middle) {\Large$c d$};
\node[boxes,above of=middle] (top) {$\displaystyle a^b$};
\node[boxes,below of=middle] (bottom) {$\displaystyle e+f+g+h^i$};
%
\node[right of=top, node distance = 35pt] (toplabel) {the top};
\draw[red] (toplabel) -- (top);
\node[left of=middle, node distance = 45pt] (midlabel) {the middle};
\draw[red] (midlabel) -- (middle);
\node[below of=bottom, node distance = 25pt] (botlabel) {the bottom};
\draw[red] (botlabel) -- (bottom);
%
\end{tikzpicture}
underbrace's look very nice, but perhaps only can be used in cases where an item is very wide (which I might use in some places). Is there some other element that matches the style ofunderbrace, but can be used in situations where I must refer to a narrower item? Or place the label to the right or left? – Village Dec 2 '11 at 3:03