Control the size of an equation (in display mode)

Question

There are several ways to show an equation. I'm trying to put a big equation in display mode (i.e.: \[ {\equation} \]) and to keep it's dimensions within the size that is reasonable, using adjustbox.

This is the minimal (non) working example:

\documentclass[a4paper,twoside,twocolumn,final]{article}
\setlength\textwidth{42.5pc}
\setlength\columnsep{1.5pc}
\usepackage{adjustbox}
\begin{document}
\texttt{\\columnwidth is \the\columnwidth}. We are going to see a wonderful equation, it extends to the other column and knows no limits:
%\adjustbox{max size={\columnwidth}{\textheight},keepaspectratio,center}{
    \[
    \frac{k + l + m + n + p + q + r + t + u + v + w + x + y + z}{a + b + c + d + f + g + h + i + j + k + l + m + \frac{t + r + y + l + k + s}{\frac{small + characters }{are + fine}}}
    \]
%}
\end{document}

I have tried a number of other combinations, like minipage, gather and some other. Either it doesn't work or there are some margins that prevent the equation from expanding to the specified width (in this case \columnwidth).

Answer 1

Just resize the equation:

\documentclass[a4paper,twoside,twocolumn,final]{article}
\usepackage{graphicx}
\setlength\textwidth{42.5pc}
\setlength\columnsep{1.5pc}
\begin{document}
\texttt{\\columnwidth is \the\columnwidth}. We are going to see a wonderful equation, it extends to the other column and knows no limits:
\[
\resizebox{\columnwidth}{!}{$\displaystyle
\frac{k + l + m + n + p + q + r + t + u + v + w + x + y + z}{a + b + c + d + f + g + h + i + j + k + l + m + \frac{t + r + y + l + k + s}{\frac{small + characters }{are + fine}}}
$}
\]
\end{document}

Answer 2

I'd give the \splitfrac macro, which is provided by the mathtools package, a try. It can be used in both the numerator and the denominator of a fraction expressions. As the following example shows, \splitfrac macros can be nested.

\documentclass[a4paper,twoside,twocolumn,final]{article}
\setlength\textwidth{42.5pc}
\setlength\columnsep{1.5pc}
\usepackage{mathtools}
\begin{document}
\texttt{\textbackslash columnwidth is \the\columnwidth}. 

We are going to see a wonderful equation, it extends to the other column and knows no limits:
\[
\frac{\splitfrac{k + l + m + n + p + q + r + t}{ + u + v + w + x + y + z}}{\splitfrac{a + b + c + d + f + g + h + i + j}{ \splitfrac{+ k + l + m + t + r + y + l + k + s }{+ small + characters + are + fine}}}
\]
\end{document}