From info latex
:
\fontsize{size}{skip}
Set font size. The first parameter is the font size to switch to; the second is the\baselineskip
to use. The unit of both parameters defaults to pt. A rule of thumb is that the baselineskip should be 1.2 times the font size.
As @frabjous noted, you'll need to add \selectfont
afterwards to make it kick in.