You can use the tocloft package and the command families \cftXfont (for the sectional number and titles) and \cftXpagefont (for the page numbers):
\documentclass[12pt]{report}
\usepackage{tocloft}
\renewcommand\cftchapfont{\LARGE\bfseries}
\renewcommand\cftsecfont{\LARGE}
\renewcommand\cftchappagefont{\LARGE\bfseries}
\renewcommand\cftsecpagefont{\LARGE}
\begin{document}
\tableofcontents
Some text in standard size just for comparison.
\addcontentsline{toc}{chapter}{Introduction}
\addcontentsline{toc}{chapter}{Factoring the Group Determinant}
\addcontentsline{toc}{section}{Finite Abelian Groups}
\addcontentsline{toc}{chapter}{References}
\end{document}
Instead of \LARGE you can use any other font size switch or \fontsize{...}{...}\selectfont.

Redefining also the family \cftXafterpnum will give you better spacing between entries:
\documentclass[12pt]{report}
\usepackage{tocloft}
\renewcommand\cftchapfont{\LARGE\bfseries}
\renewcommand\cftsecfont{\LARGE}
\renewcommand\cftchappagefont{\LARGE\bfseries}
\renewcommand\cftsecpagefont{\LARGE}
\renewcommand\cftchapafterpnum{\par\addvspace{6pt}}
\renewcommand\cftsecafterpnum{\par\addvspace{6pt}}
\begin{document}
\tableofcontents
Some text in standard size just for comparison.
\addcontentsline{toc}{chapter}{Introduction}
\addcontentsline{toc}{chapter}{Factoring the Group Determinant}
\addcontentsline{toc}{section}{Finite Abelian Groups}
\addcontentsline{toc}{chapter}{References}
\end{document}
