[Tex/LaTex] Set font size for footnotes

fontsizefootnotes

I am writing my thesis and my university requirements ask me to set the footnotes font size at 9pt. The body text size is 12pt. I tried to change the value using :

\renewcommand{\footnotesize}{(one of the 8 values)}

but looking at this table
enter image description here

I realized that none of the possible values would give me a font size of 9. How can I solve the problem?

Here it is the preamble:

\documentclass[a4paper,12pt]{report}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\let\up\textsuperscript
\usepackage{blindtext}
\usepackage{setspace}
\renewcommand{\baselinestretch}{1.5} 
\usepackage{pslatex}


\usepackage{titlesec}
\titleformat{\chapter}
  {\normalfont\fontsize{14}{14}\bfseries}{\thechapter}{1em}{}
\titleformat{\section}
  {\normalfont\fontsize{13}{13}\bfseries}{\thesection}{1em}{}
\titleformat{\subsection}
  {\normalfont\fontsize{12}{12}\bfseries\slshape}{\thesubsection}{1em}{}
\usepackage{amsmath, amsthm, amssymb, amsfonts}
\mathchardef\mhyphen="2D



\usepackage{fancyhdr}

\fancypagestyle{IHA-fancy-style}{%
  \fancyhf{}% Clear header and footer
  \fancyhead[LE,RO]{\slshape \rightmark}
  \fancyhead[LO,RE]{\slshape \leftmark}
  \fancyfoot[R]{\thepage}% Custom footer
  \renewcommand{\headrulewidth}{0pt}% Line at the header visible
  \renewcommand{\footrulewidth}{0pt}% Line at the footer visible
}

% Redefine the plain page style
\fancypagestyle{plain}{%
  \fancyhf{}%
  \fancyfoot[R]{\thepage}%
  \renewcommand{\headrulewidth}{0pt}% Line at the header invisible
  \renewcommand{\footrulewidth}{0pt}% Line at the footer visible
}

\usepackage{fancyhdr}
\pagestyle{plain}
\lhead{}
\chead{}
\rhead{}
\lfoot{}
\cfoot{}
\rfoot{\thepage}
\renewcommand{\headrulewidth}{0pt}
\renewcommand{\footrulewidth}{0pt}

\usepackage{graphicx}
\usepackage{color}
\usepackage{transparent}

\usepackage{caption}

\usepackage{enumerate}
\usepackage{url}


\usepackage{natbib}
\bibliographystyle{abbrvnat}
\setcitestyle{authoryear,open={},close={}}
\usepackage[]{tocbibind}

\usepackage{makeidx}


\usepackage{lipsum}

%\pagestyle{headings}
%\usepackage{emptypage}


\title{Epistemic Contextualism:\\ Skeptcism and Intuitions}
\author{Alessandro Rizzo\\ Department of Philosophy, Vita-Salute S.Raffaele University}

Best Answer

Copy the definition of \footnotesize from size11.clo:

\makeatletter
\renewcommand\footnotesize{%
   \@setfontsize\footnotesize\@ixpt{11}%
   \abovedisplayskip 8\p@ \@plus2\p@ \@minus4\p@
   \abovedisplayshortskip \z@ \@plus\p@
   \belowdisplayshortskip 4\p@ \@plus2\p@ \@minus2\p@
   \def\@listi{\leftmargin\leftmargini
               \topsep 4\p@ \@plus2\p@ \@minus2\p@
               \parsep 2\p@ \@plus\p@ \@minus\p@
               \itemsep \parsep}%
   \belowdisplayskip \abovedisplayskip
}
\makeatother

This code should go in the preamble, say after the loading of packages.

Just changing the values for \fontsize is wrong, as footnotes can contain also lists or math, in general.

Notes about your code

The pslatex package is obsolete. If you want (or need) to use Times, prefer

\usepackage{newtxtext,newtxmath}

Stating \fontsize{14}{14} for chapter titles is dubious, because titles spanning two lines will be awkwardly set. Similarly for the other levels.

Instead of \renewcommand{\baselinestretch}{1.5}, use the setspace package.

What's this about? The report and article classes read one among size10.clo, size11.clo and size12.clo, depending on the font size option passed to the class (default 10pt).

Such files contain definitions and settings for the commands and parameters that depend on font size. In particular they define \normalsize, \small, \footnotesize, \large and so on, but also set values for \parindent, \bigskipamount, \textwidth and several other parameters (most notably those that pertain to lists).

The duty of \footnotesize, for example, is to set the font size and the baseline skip, but also the parameters for the vertical space before and after math displays and for spacings in first level lists).

By borrowing the definition of \footnotesize from size11.clo, where footnotes are typeset at 9pt, we get what we need.

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