How do I set the size of Emacs’ window?

If you want to change the size according to resolution you can do something like this (adjusting the preferred width and resolutions according to your specific needs):

(defun set-frame-size-according-to-resolution ()
  (interactive)
  (if window-system
  (progn
    ;; use 120 char wide window for largeish displays
    ;; and smaller 80 column windows for smaller displays
    ;; pick whatever numbers make sense for you
    (if (> (x-display-pixel-width) 1280)
           (add-to-list 'default-frame-alist (cons 'width 120))
           (add-to-list 'default-frame-alist (cons 'width 80)))
    ;; for the height, subtract a couple hundred pixels
    ;; from the screen height (for panels, menubars and
    ;; whatnot), then divide by the height of a char to
    ;; get the height we want
    (add-to-list 'default-frame-alist 
         (cons 'height (/ (- (x-display-pixel-height) 200)
                             (frame-char-height)))))))

(set-frame-size-according-to-resolution)

Note that window-system is deprecated in newer versions of emacs. A suitable replacement is (display-graphic-p). See this answer to the question How to detect that emacs is in terminal-mode? for a little more background.