minibuffer-frame-alist

minibuffer-frame-alist is a variable defined in `frame.el'.
Its value is

((width . 80) (height . 2))

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

Documentation:
Alist of parameters for the initial minibuffer frame.
This is the minibuffer frame created if `initial-frame-alist'
calls for a frame without a minibuffer. The parameters specified
here supersede those given in `default-frame-alist', for the
initial minibuffer frame.

You can set this in your init file; for example,

(setq minibuffer-frame-alist
'((top . 1) (left . 1) (width . 80) (height . 2)))

It is not necessary to include (minibuffer . only); that is
appended when the minibuffer frame is created.

You can customize this variable.