Martin's answer works well up to a limited size, but TeX has an upper limit on every length, including the page height. This limit is fairly small, equivalent to only around 20 sheets of letter paper placed end to end. To work around this limitation, I scaled everything in the document down by 40 times, then used pdfposter to scale it back up after compiling the document.
\newcommand{\pagescale}{0.025} % 1/40
% Use these instead of the usual units.
\newlength{\PT}\setlength{\PT}{\pagescale pt}
\newlength{\IN}\setlength{\IN}{\pagescale in}
\newlength{\CM}\setlength{\CM}{\pagescale cm}
\newlength{\MM}\setlength{\MM}{\pagescale mm}
% LaTeX defaults are almost all defined using \p@, which is supposed to be 1pt.
\makeatletter
\setlength{\p@}{1\PT}
\makeatother
% Reset separations that were coded with pt not \p@.
\setlength{\jot}{3\PT} % Math display line separation.
\scriptspace=0.5\PT
\hfuzz=0.1\PT
\vfuzz=0.1\PT
\overfullrule=5\PT
\maxdepth=4\PT
\delimitershortfall=5\PT
\nulldelimiterspace=1.2\PT
\scriptspace=0.5\PT
\parindent=20\PT
\parskip=0\PT plus 1\PT
\abovedisplayskip=12\PT plus 3\PT minus 9\PT
\abovedisplayshortskip=0\PT plus 3\PT
\belowdisplayskip=12\PT plus 3\PT minus 9\PT
\belowdisplayshortskip=7\PT plus 3\PT minus 4\PT
\topskip=10\PT
\splittopskip=10\PT
\parfillskip=0\PT plus 1fil
\normalbaselineskip=12\PT
\normallineskip=1\PT
\normallineskiplimit=0\PT
\smallskipamount=3\PT plus 1\PT minus 1\PT
\medskipamount =6\PT plus 2\PT minus 2\PT
\bigskipamount =12\PT plus 4\PT minus 4\PT
\def\fontsubfuzz{.4\PT}
\unitlength = 1\PT
\fboxsep = 3\PT
\fboxrule = .4\PT
\let\Oldhrule\hrule
\def\hrule{\Oldhrule height 0.4\PT}
\let\Oldvrule\vrule
\def\vrule{\Oldvrule width 0.4\PT}
\documentclass{article}
\renewcommand{\baselinestretch}{\pagescale}
\newcommand{\marginsize}{2\CM}
\usepackage[paperwidth=8.5\IN,paperheight=16383pt,margin=\marginsize]{geometry}
\usepackage[active,tightpage]{preview}
\renewcommand{\PreviewBorder}{\marginsize}
% No point in page numbers if there is only one page.
\pagenumbering{gobble}
% Need to rerun math font definitions to get the sizes right. Loading amsfonts
% is sufficient.
\usepackage{amsfonts}
\usepackage{amsmath,lipsum}
\begin{document}
\begin{preview}
\begin{minipage}[b]{\textwidth}
\LaTeX
$\int_0^1 x^2 dx = \frac13$. \footnote{A footnote}
\hrule height \PT width 100\PT
\begin{align}
\int_{-\infty}^\infty e^{-x^2} dx &= \sqrt{\pi} \
z &= w
\end{align}
\vrule height 100\PT
\lipsum
{\huge \lipsum
}
{\tiny \lipsum
}
% Very long example text, which couldn't fit on a single page at normal scale.
\newcount\foo
\foo=1000
\loop
\lipsum[1]\par
\advance \foo -1
\ifnum \foo>0
\repeat
\end{minipage}
\end{preview}
\end{document}
If you have floats then you likely want to follow Andy's advise and set \setcounter{totalnumber}{100}, so that LaTeX can put more of them on a page.
The following script uses pdfposter to scale the pdf back to its usual dimensions after it has been built.
#!/bin/bash
if [ $# -ne 3 ]
then
echo "Usage: $0 <scale factor> <input> <output>"
exit 1
fi
scale=$1
newpagesize=$(pdfinfo "$2" | awk '/Page size:/ { print ('"$scale"' * $3) "x" ('"$scale"' * $5) "pt" }')
pdfposter "$2" -s "$scale" -m "$newpagesize" $3
minipagetakes care of the footnotes (or would that be more restrictive than necessary?) – pascal May 27 '11 at 21:43minipages. However they number footnotes differently. Floats won't work either. – Martin Scharrer May 27 '11 at 22:25\begin{figure}[h]...but that still doesn't work because of "Not in outer par mode". Any idea of getting around that, and getting an image with a caption? – Supernormal Sep 02 '16 at 13:48standalonepackage which I created since I wrote this post. It is either based onpreviewor uses its own technique, depending on the used options. It can also be used to turn floats into non-floats automatically (but I'm not sure if I published that feature already). – Martin Scharrer Sep 09 '16 at 08:10