LaTeX - Changing the font size for a document, but in the preamble, not the document class?

will be helpful for those in need Stage 1 - Installing IOP Document Files for Texlive (Or Miktex)
Windows Users: Check this link for directory location info: http://wiki.lyx.org/Layouts/Iopart

it helps some times You should have some quotes on detail. and try detail.style.fontSize instead of detail.style["font-size"]

var detail = document.getElementById("detail");
detail.style.fontSize = "100%";

With these it helps I think I figured it out:
My problem was that while using RMarkdown the options tikzDocumentDeclaration, tikzLatexPackages ... (nearly all options for tikzDevice) were not set automatically. When you use knitr the options for tikzDevice get set up in the process of splitting up markup and code chunks from the source file. With RMarkdown there is no LaTeX code to extract and use with tikz because pandoc generates it after the graphic is rendered. So one can either define the tikz... options manually or use the chunk option external=FALSE like user2554330 suggested.

\documentclass[fontsize=12pt]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\begin{document}

<<r plot, dev='tikz', echo=FALSE>>=
plot(rnorm(50))
@

\end{document}