Adjust all the font size changing macros to mean \normalsize:
\documentclass[12pt]{report}
\renewcommand{\tiny}{\normalsize}
\renewcommand{\footnotesize}{\normalsize}
\renewcommand{\small}{\normalsize}
\renewcommand{\large}{\normalsize}
\renewcommand{\Large}{\normalsize}
\renewcommand{\LARGE}{\normalsize}
\renewcommand{\huge}{\normalsize}
\renewcommand{\Huge}{\normalsize}
\usepackage{fancyhdr,blindtext}
\pagestyle{fancy}
\title{A Title}
\author{An Author}
\date{\today}
\begin{document}
\maketitle
\tableofcontents
\Blinddocument
\end{document}
For the most part, this will ensure that all font size switches are "no-ops". Direct font switches using \fontsize is still in play.
Note that this does not change any of the math font sizes. However, that wasn't part of the request and it would horrible.
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The pslatex package is obsolete.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.
Best Answer
The correct way is
\documentclass[sigconf,10pt]{acmart}. This is the official answer I get from the conference chairs.