The only way to fit a long equation, that I can think of, without additional work as I mentioned in the comment above, is to simply reduce the size of the latex math font to use for this one equation.
It is easy to modify the font size in Latex. One command can do it. Hence you'd need to edit the Latex file, but you have to do that any way to add the standard Latex headers to build the TexForm expression, and it is only one command.
Here is an example:
Make a very long equation that will overflow in Latex since it is not breakable automatically by Latex:
Remove["Global`*"]
res = (Sum[Sin[a x], {a, 1, 15}])/(Sum[Cos[b x], {b, 1, 15}])
Export it to Latex as is first:
SetDirectory[NotebookDirectory[]];
Export["res.tex", TeXForm[res], "Text"];
Open the file res.tex, and add the Latex \begin{document}....
needed to compile it
around the code exported by Mathematica. This is the result
\documentclass[12pt]{article}
\begin{document}
\begin{equation}% this below was generated by Mathematica export
\frac{\sin (x)+\sin (2 x)+\sin (3 x)+\sin (4 x)+\sin (5 x)+\sin (6 x)+\sin (7 x)+\sin (8 x)+\sin (9 x)+\sin (10 x)+\sin (11 x)+\sin (12 x)+\sin (13 x)+\sin (14 x)+\sin (15 x)}{\cos (x)+\cos (2 x)+\cos (3 x)+\cos (4 x)+\cos (5 x)+\cos (6 x)+\cos (7 x)+\cos (8 x)+\cos (9 x)+\cos (10 x)+\cos (11 x)+\cos (12 x)+\cos (13 x)+\cos (14 x)+\cos (15 x)}
\end{equation}
\end{document}
Compile it and looking at the pdf, it overflows as expected:
>pdflatex res.tex
This is pdfTeX, Version 3.1415926-2.5-1.40.14 (TeX Live 2013)
restricted \write18 enabled.
Now, do the same again, but make the math font \scriptscriptstyle
, this requires just adding this one command as follows
\documentclass[12pt]{article}
\begin{document}
\begin{equation}
{\scriptscriptstyle \frac{\sin (x)+\sin (2 x)+\sin (3 x)+\sin (4 x)+\sin (5 x)+\sin (6 x)+\sin (7 x)+\sin (8 x)+\sin (9 x)+\sin (10 x)+\sin (11 x)+\sin (12 x)+\sin (13 x)+\sin (14 x)+\sin (15 x)}{\cos (x)+\cos (2 x)+\cos (3 x)+\cos (4 x)+\cos (5 x)+\cos (6 x)+\cos (7 x)+\cos (8 x)+\cos (9 x)+\cos (10 x)+\cos (11 x)+\cos (12 x)+\cos (13 x)+\cos (14 x)+\cos (15 x)}
}
\end{equation}
\end{document}
Now compile again:
Now it fits. There are other way to change the math font. You can make it even smaller if needed. Since it is in PDF, that is not an issue, as one can always increase the magnification inside PDF to see the whole equation, but now at least, it will not be cut off as before.
If you want one command to reduce the font size of all the math in the document, without having to do it for each equation, then \DeclareMathSizes
can be used, here is an example:
\documentclass[12pt]{article}
\usepackage{amsmath}
\DeclareMathSizes{12}{5}{5}{5}
\begin{document}
\begin{equation}% this below was generated by Mathematica export
\frac{\sin (x)+\sin (2 x)+\sin (3 x)+\sin (4 x)+\sin (5 x)+\sin (6 x)+\sin (7 x)+\sin (8 x)+\sin (9 x)+\sin (10 x)+\sin (11 x)+\sin (12 x)+\sin (13 x)+\sin (14 x)+\sin (15 x)}{\cos (x)+\cos (2 x)+\cos (3 x)+\cos (4 x)+\cos (5 x)+\cos (6 x)+\cos (7 x)+\cos (8 x)+\cos (9 x)+\cos (10 x)+\cos (11 x)+\cos (12 x)+\cos (13 x)+\cos (14 x)+\cos (15 x)}
\end{equation}
\end{document}
update
Just wanted to add one another thought:
I would consider bypassing this whole approach, and simply export those large equations directly to pdf from Mathematica.
When exporting to pdf from Mathematica, this problem goes way. Then from Latex use \usepackage{pdfpages}
to insert those pdf page into the pdf being build by pdflatex from the main Latex document.
Sure, the layout would not look exactly the same as the other Latex pages, and it depends what you are doing. If this is just a report and you want to include some equations, this would work. If this was an official paper or such, this might not work. But I thought to mention this as a possible way.
\frac{very very long line}{x}
then there is nothing but manual edit that can break it, including manual splitting if needed. The answer to your question is: There is no solution to this within Mathematica $\endgroup$