\documentclass[12pt]{article}
\usepackage{parskip}
\pagestyle{empty}
\begin{document}
here's 12pt Computer Modern

{\font\cmbig=cmr5 at12pt\cmbig here's 5pt Computer Modern scaled up to
  12pt}

{\font\cmsmall=cmr17 at12pt\cmsmall here's 17pt Computer Modern scaled
  down to 12pt} 
\end{document}