documentclass[9pt,t]{beamer}

usetheme{Berkeley}
usepackage{thumbpdf}
usepackage{wasysym}
usepackage{ucs}
usepackage[utf8]{inputenc}
usepackage{pgf,pgfarrows,pgfnodes,pgfautomata,pgfheaps,pgfshade}
usepackage{verbatim}
usepackage[utf8]{inputenc}
usepackage{listings}
usepackage{color}
usepackage{amsfonts}
usepackage{amsmath,amssymb,epsfig,color}
usepackage{psfrag,subfigure}
usepackage{times}
%usepackage{textcomp}
%usepackage{eurosym}
usepackage{amsthm,amsmath,amsfonts,mathbbol,mathrsfs,stmaryrd,textcomp}

begin{document}

begin{frame}[t]
    huge{I want this text to be large}

    bigskip
But this text is also appearing as large. Not what I want!

end{frame}
end{document}

documentclass[9pt,t]{beamer}

usetheme{Berkeley}
usepackage{thumbpdf}
usepackage{wasysym}
usepackage{ucs}
usepackage[utf8]{inputenc}
usepackage{pgf,pgfarrows,pgfnodes,pgfautomata,pgfheaps,pgfshade}
usepackage{verbatim}
usepackage[utf8]{inputenc}
usepackage{listings}
usepackage{color}
usepackage{amsfonts}
usepackage{amsmath,amssymb,epsfig,color}
usepackage{psfrag,subfigure}
usepackage{times}
%usepackage{textcomp}
%usepackage{eurosym}
usepackage{amsthm,amsmath,amsfonts,mathbbol,mathrsfs,stmaryrd,textcomp}

begin{document}

begin{frame}[t]
    %inside the brace to limit
    {huge I want this text to be large

    bigskip}% always make sure end of para is in scope for a size change.
But this text is also appearing as large. Not what I want!

end{frame}
end{document}

documentclass[9pt,t]{beamer}

usetheme{Berkeley}
usepackage{thumbpdf}
usepackage{wasysym}
usepackage{ucs}
usepackage[utf8]{inputenc}
usepackage{pgf,pgfarrows,pgfnodes,pgfautomata,pgfheaps,pgfshade}
usepackage{verbatim}
usepackage[utf8]{inputenc}
usepackage{listings}
usepackage{color}
usepackage{amsfonts}
usepackage{amsmath,amssymb,epsfig,color}
usepackage{psfrag,subfigure}
usepackage{times}
%usepackage{textcomp}
%usepackage{eurosym}
usepackage{amsthm,amsmath,amsfonts,mathbbol,mathrsfs,stmaryrd,textcomp}

begin{document}

begin{frame}[t]
    % Huge text after this
    huge{I want this text to be large}

    bigskip
    % go back to normalsize here
    normalsize{But this text is also appearing as large. Not what I want!}

end{frame}
end{document}