Emacs set font-size based on monitor size.

init.el

    ;; set some defaults 
    (setq my-font-name "monospace")
    (setq my-font-size 10)
    
    ;; this method seems to work on gui systems only 
    (when window-system
      (if  (eq system-type 'darwin)
        (setq my-font-name "Meslo"))
      (if   (eq system-type 'gnu/linux)
        (setq my-font-name "FiraCode"))
    
      (if (> (x-display-pixel-width) 1600)
        (setq my-font-size 16))
      (if (<= (x-display-pixel-width) 1600)
        (setq my-font-size 11)))
   
    ;; set the font
    (set-frame-font (format "%s %d" my-font-name my-font-size))