default-frame-alist is a variable defined in `C source code'.
Its value is

  • This variable may be risky if used as a file-local variable.

Alist of default values for frame creation.
These may be set in your init file, like this:
(setq default-frame-alist '((width . 80) (height . 55) (menu-bar-lines . 1)))
These override values given in window system configuration data,
including X Windows' defaults database.
For values specific to the first Emacs frame, see `initial-frame-alist'.
For window-system specific values, see `window-system-default-frame-alist'.
For values specific to the separate minibuffer frame, see
The `menu-bar-lines' element of the list controls whether new frames
have menu bars; `menu-bar-mode' works by altering this element.
Setting this variable does not affect existing frames, only new ones.

You can customize this variable.