Altering fontsize in equations globally

Question

I am trying to typeset all formulas in an article using \small font.

If I use \DeclareMathSizes{9}{8}{6}{4}, then the size of in-line math in regular text is also changed. However, I want to redefine the font size only for multi-line equations wrapped in an align environment.

I also tried to redefine the align command in the following way:

\let\oldalign=\align
\renewcommand{\align}{\small\oldalign}

This results in some text paragraphs after align environments now being typeset in \small font, although well outside the align environment. Sorry, I don't have a minimal example right now. It might have to do with the cls template I'm using (sig-alt-full).

I tried adding a \normalsize as a fix, without success:

\let\oldalign=\align
\renewcommand{\align}{\small\oldalign\normalsize}

I then proceeded to defining my own align environment, as follows:

\newenvironment{myalign}
{% begin code
\begin{small}
\begin{align}
}
{% end code
\end{align}
\end{small} }

This one fails because, according to LaTeX, \begin{small} is ended by \end{document}.

Answer 1

I don't think it's really a good idea. However the following seems to work:

\documentclass{article}
\usepackage{amsmath,lipsum}
\newenvironment{myalign}{\par\nobreak\small\noindent\align}{\endalign}
\begin{document}
\lipsum*[2]
\begin{myalign}
a&=b\\
c&=d
\end{myalign}
\lipsum[2]

\renewenvironment{myalign}{\par\nobreak\tiny\noindent\align}{\endalign}

\lipsum*[2]
\begin{myalign}
a&=b\\
c&=d
\end{myalign}
\lipsum[2]

\end{document}

The redefinition is just to show more clearly that it works (small size is not so different from normal size).

---

If you want to use the original align and align* environments, then the following trick will do

\usepackage{etoolbox}
\preto\align{\par\nobreak\small\noindent}
\expandafter\preto\csname align*\endcsname{\par\nobreak\small\noindent}

Similar tricks can be used for the other alignment environments.

Full example:

\documentclass{article}
\usepackage{amsmath,lipsum}

\usepackage{etoolbox}
\preto\align{\par\nobreak\small\noindent}
\expandafter\preto\csname align*\endcsname{\par\nobreak\small\noindent}

\begin{document}
\lipsum*[2]
\begin{align}
a&=b\\
c&=d
\end{align}
\lipsum*[2]
\begin{align*}
a&=b\\
c&=d
\end{align*}
\lipsum[2]

\end{document}