Set font size for footnotes
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The pslatex package is obsolete.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.
You can use arbitrary font sizes within LaTeX, not only the default ones (\normalsize, \Large, \footnote size, &c.). The command for a font size of 9pt would be:
\renewcommand{\footnotesize}{\fontsize{9pt}{11pt}\selectfont}
The second value is value for the interline skip for that size. Usually it is about 20 % more than the nominal size.
If the restrictions appply to only the footnotes, and nothing more, you can set it just for the footnotes. Here an example using KOMA-script and a ridiculously high fontsize just to make everything more obvious.
\documentclass[fontsize=22pt]{scrartcl}
\usepackage{lmodern}
\addtokomafont{footnote}{\fontsize{9pt}{11pt}\selectfont}
\setlength{\textheight}{5cm}
\makeatletter
\newcommand{\fnsize}{current size: \f@size\,pt}
\makeatother
\begin{document}
Capybara went crazy\footnote{Charles Seymoure Capybara was crazy
all his life, no need to worry. \fnsize}
{\footnotesize \fnsize}
\end{document}
KOMA-script uses a fallback calculation of the relevant sizes, hence the very odd value of 18.33356pt.
We can do the very same for standard report, but it requires patching the internal commands of the LaTeX kernel. The resuult though, is the same; a change of font size for just the footnotes.
\documentclass[12pt]{report}
\usepackage{lmodern}
\usepackage{etoolbox}
%\tracingpatches
\makeatletter
\patchcmd{\@footnotetext}{\footnotesize}{\fontsize{9pt}{11pt}\selectfont}{}{}
\makeatother
\setlength{\textheight}{5cm}
\makeatletter
\newcommand{\fnsize}{current size:~\f@size\,pt}
\makeatother
\begin{document}
Capybara went crazy\footnote{Charles Seymoure Capybara was crazy
all his life, no need to worry. \fnsize}
{\footnotesize \fnsize}
\end{document}