LaTeX -- specify font point size?

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.

There are a couple of LaTeX packages available that provide for additional font sizes:

The moresize package offers additional commands to have a broader range (\HUGE, \ssmall), whereas extsizes extends the standard LaTeX classes by additional class options (8pt, ... 20pt for the base font, so that all fonts in the document become smaller or bigger, but remain their relative difference. Advanced document classes (memoir, but especially the KOMA-Skript bundle) provide built-in capabilities for flexible font sizes.