Latex

The LaTeX and EMTeX driver allows one to specify a font type and a font size for the labels around a gnuplot graph.

Options are: Fonts: 

      default          (Roman 10 point)
      courier
      roman

at any size you specify. (BEWARE METAFONT will not like odd sizes.) eg. 

      gnuplot> set term latex courier 5

Unless your driver is capable of building fonts at any size (e.g. dvips), stick to the standard 10, 11 and 12 point size.