If you use the standard notebook front end for
Mathematica, then you can set
BaseStyle to be the name of a style defined in your current notebook's stylesheet. You can also explicitly specify how text should be formatted by using options such as
FontSize and
FontFamily. Note that
FontSize gives the absolute size of the font to use, measured in units of printer's points, with one point being

inches. If you resize a plot whose font size is specified as a number, the text in it will not by default change size: to get text of a different size you must explicitly specify a new value for the
FontSize option. If you resize a plot whose font size is specified as a scaled quantity, the font will scale as the plot is resized. With
FontSize->Scaled[s], the effective font size will be
s scaled units in the plot.
You should realize that the ability to refer to styles such as
"Section" depends on using the standard
Mathematica notebook front end. Even if you are just using a text-based interface to
Mathematica, however, you can still specify formatting of text in graphics using options such as
FontSize. The complete collection of options that you can use is given in
"Text and Font Options".