To startup Emacs maximised, add to your init file:

;; Start maximised (cross-platf)
(add-hook 'window-setup-hook 'toggle-frame-maximized t)

In case you want to go full screen or, as they say now, distraction free mode:

;; Start fullscreen (cross-platf)

(add-hook 'window-setup-hook 'toggle-frame-fullscreen t)

NOTE: This works in Windows too.

To startup Emacs maximised, add to your init file:

;; Start maximised (cross-platf)
(add-hook 'window-setup-hook 'toggle-frame-maximized t)

In case you want to go full screen or, as they say now, distraction free mode:

;; Start fullscreen (cross-platf)
(add-hook 'window-setup-hook 'toggle-frame-fullscreen t)

NOTE: This works in Windows too.