How to set the font size in Emacs?

(set-face-attribute 'default nil :height 100)

The value is in 1/10pt, so 100 will give you 10pt, etc.

Press Shift and the first mouse button. You can change the font size in the following way: This website has more detail.

M-x customize-face RET default will allow you to set the face default face, on which all other faces base on. There you can set the font-size.

Here is what is in my .emacs. actually, color-theme will set the basics, then my custom face setting will override some stuff. the custom-set-faces is written by emacs's customize-face mechanism:

;; my colour theme is whateveryouwant :)
(require 'color-theme)
(color-theme-initialize)
(color-theme-whateveryouwant)

(custom-set-faces
  ;; custom-set-faces was added by Custom.
  ;; If you edit it by hand, you could mess it up, so be careful.
  ;; Your init file should contain only one such instance.
  ;; If there is more than one, they won't work right.
 '(default ((t (:stipple nil :background "white" :foreground "black" :inverse-video nil :box nil :strike-through nil :overline nil :underline nil :slant normal :weight normal :height 98 :width normal :foundry "unknown" :family "DejaVu Sans Mono"))))
 '(font-lock-comment-face ((t (:foreground "darkorange4"))))
 '(font-lock-function-name-face ((t (:foreground "navy"))))
 '(font-lock-keyword-face ((t (:foreground "red4"))))
 '(font-lock-type-face ((t (:foreground "black"))))
 '(linum ((t (:inherit shadow :background "gray95"))))
 '(mode-line ((t (nil nil nil nil :background "grey90" (:line-width -1 :color nil :style released-button) "black" :box nil :width condensed :foundry "unknown" :family "DejaVu Sans Mono")))))

I've got the following in my .emacs:

(defun fontify-frame (frame)
  (set-frame-parameter frame 'font "Monospace-11"))

;; Fontify current frame
(fontify-frame nil)
;; Fontify any future frames
(push 'fontify-frame after-make-frame-functions) 

You can subsitute any font of your choosing for "Monospace-11". The set of available options is highly system-dependent. Using M-x set-default-font and looking at the tab-completions will give you some ideas. On my system, with Emacs 23 and anti-aliasing enabled, can choose system fonts by name, e.g., Monospace, Sans Serif, etc.

Here's a snippet that lets you directly specify the global font size using an interactive function:

(defun set-font-size ()
    "Set the font size."
  (interactive)
  (set-face-attribute
   'default nil :height
   (string-to-number
    (read-string "Font size: " (number-to-string (face-attribute 'default :height nil))))))