Change base fontsize for rest of document

Question

There are already two questions regarding this problem:

• Switching to different font size(s) locally
• Change font size in the middle of a document or for individual included .tex file [duplicate]

The problem with the first approach is that it adds too much page margin and that the content of the Appendix is off.

The problem with the second approach (from the comments to the question) is that it only affects the font size of the normal text, not that of headings etc.

I'd like a solution like the first, but it should keep the page margins as they are.

It should work with the standard document class article.

Using additional packages is not a problem, but they shouldn't alter the defined style.

Minimal Example

\documentclass[12pt,a4paper]{article}
\special{papersize=210mm,297mm}
\setlength{\parindent}{0pt}
\setlength{\parskip}{5pt plus 2pt minus 1pt}
\begin{document}
  Normal sized text, headings, etc.
  \begin{appendices}
    Smaller sized text, headings, etc.
  \end{appendices}
\end{document}

Answer 1

With the KOMA-classes it can be easily done:

\documentclass[
 a4paper,
 fontsize=12pt,
 parskip=half
]{scrreprt}

\usepackage{lipsum}

\begin{document}
\lipsum[1]

{\KOMAoptions{fontsize=10pt} \lipsum[1] \small abc\par}

\lipsum[1] \small abc

\end{document}