Changing text font size

I have been trying without any luck to change the font size on my
slides. I tried \ptsize{n} , n = 10 and 12. First before
\begin{document} and then after. Nothing happens. Is there a way of
doing this? The header line of my file is:
\documentclass[ps,blends,final,slideBW,nocolorBG,total]{prosper}

Thank you.
Nkgau



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