To maximize or fullscreen Emacs at startup, add one of the following to ~/.doom.d/config.el:
(add-to-list 'initial-frame-alist '(fullscreen . maximized))
(add-hook 'window-setup-hook #'toggle-frame-maximized)
(add-hook 'window-setup-hook #'toggle-frame-fullscreen)
Each method has slightly different effects that vary from OS to OS. You’ll have to decide for yourself which you prefer.