To change the default font of EXMH, the following two lines should be added to your .exmh-defaults file.
*Font: *Courier-medium-r-*-*-14*
*fl_font: *Courier-medium-r-*-*-14*
These two lines should be added between the lines where it says "Lines below here added automatically" and "Do not edit below this line" respectively. A list of available fonts can be found by typing xlsfonts at the command prompt. The fonts listed by the xlsfonts command can be substituted to the *Courier-medium-r-*-*-14*lines that you have added to your .exmh-defaults file.