scaling font size in single equation

Question

I am dealing with the proofs of my academic paper, which is dealt with by some company who use LaTeX only for setting each equation individually. In order to adapt the font size in an equation (let's not discuss here why) I could use commands like \small, i.e.

\small
\begin{eqnarray*}
    a=b.
\end{eqnarray*}

but is there a way to scale the fontsize to an arbitrary size? Note that I cannot use any packages, but standard LaTeX commands work. There must be a way, since I have seen it done in a the proofs of another paper, but cannot remember how. (Internet searches only bring up solutions with commands like \small or some LaTeX package.)

I attempted

\scalebox{0.98}{
\begin{eqnarray*}
    a=b.
\end{eqnarray*}
}

but got missing \endgroup (from the LaTeX interface used for proof editing).

Answer 1

To expand on Jasper's comment, you could try:

\documentclass[12pt]{article}
    \usepackage[a4paper]{geometry}
    \usepackage{amsmath}
\begin{document}
\begin{eqnarray*}
    a=b.
\end{eqnarray*}
\fontsize{14pt}{14pt}\selectfont
\begin{eqnarray*}
    a=b.
\end{eqnarray*}
\fontsize{7pt}{7pt}\selectfont
\begin{eqnarray*}
    a=b.
\end{eqnarray*}
\end{document} 

Answer 2

\scalebox is like \mbox so only allows horizontal mode constructs.

so

\scalebox{0.98}{$\displaymode a=b$}

would work, but to use a multiline display you need to nest a \parbox

\scalebox{0.98}{\parbox{\textwidth}{%
\begin{align*}
a&=b\\
1&=2
\end{align*}}}