You can specify the parameters for the initial startup frame by
setting initial-frame-alist in your init file (see The Init File).
This variable’s value is an alist of parameter values used when creating the initial frame. You can set this variable to specify the appearance of the initial frame without altering subsequent frames. Each element has the form:
(parameter . value)
Emacs creates the initial frame before it reads your init
file. After reading that file, Emacs checks initial-frame-alist,
and applies the parameter settings in the altered value to the already
created initial frame.
If these settings affect the frame geometry and appearance, you’ll see the frame appear with the wrong ones and then change to the specified ones. If that bothers you, you can specify the same geometry and appearance with X resources; those do take effect before the frame is created. See X Resources in The GNU Emacs Manual.
X resource settings typically apply to all frames. If you want to
specify some X resources solely for the sake of the initial frame, and
you don’t want them to apply to subsequent frames, here’s how to achieve
this. Specify parameters in default-frame-alist to override the
X resources for subsequent frames; then, to prevent these from affecting
the initial frame, specify the same parameters in
initial-frame-alist with values that match the X resources.
If these parameters include (minibuffer . nil), that indicates
that the initial frame should have no minibuffer. In this case, Emacs
creates a separate minibuffer-only frame as well.
This variable’s value is an alist of parameter values used when
creating an initial minibuffer-only frame (i.e., the minibuffer-only
frame that Emacs creates if initial-frame-alist specifies a
frame with no minibuffer).
This is an alist specifying default values of frame parameters for all Emacs frames—the first frame, and subsequent frames. When using the X Window System, you can get the same results by means of X resources in many cases.
Setting this variable does not affect existing frames. Furthermore, functions that display a buffer in a separate frame may override the default parameters by supplying their own parameters.
If you invoke Emacs with command-line options that specify frame
appearance, those options take effect by adding elements to either
initial-frame-alist or default-frame-alist. Options
which affect just the initial frame, such as ‘--geometry’ and
‘--maximized’, add to initial-frame-alist; the others add
to default-frame-alist. see Command Line
Arguments for Emacs Invocation in The GNU Emacs Manual.