Советую использовать emacs-snapshot-gtk, если ты по какой-то причине используешь другой.
В меню Options -> Set Default Font ставишь нужный.
Для старых версий делалось так, на выбор:

(1) Add the following line to your ~/.Xdefaults:
    Emacs*font: (your-font-name)
    For example,
    Emacs*font: -misc-fixed-medium-r-normal--15-140-75-75-c-90-iso8859-1
    Note: reinstalling .Xdefaults will erase this setting.
(2) Add the following line to your ~/.emacs:
    
    (if-not-xemacs (set-default-font "your-font-name"))
    where your-font-name is your font's name as in (1).
    Note: reinstalling .emacs will erase this setting.
(3) In Emacs, type M-x, customize-face (ENTER) default (ENTER), then
    check the options you want to set and set them. After that, you can
    use the State menu in the page to apply the values for the current
    session or all future sessions. (If you want to change the font only
    for the current session, we suggest using option (4) below, as it's
    easier).
    Note: M-x stands for Meta-x, which on most computers
 means Alt-x.
    Note 2: setting for future sesssions are saved in the file
    .emacs-options in your home directory. Reinstalling this file will
    erase the setting.

Но лучше использовать то, что я вверху указал. emacs-snapshot-gtk использует сглаживание шрифтов.