Here’s how to examine the data in an X-style window geometry specification:
The function x-parse-geometry converts a standard X window
geometry string to an alist that you can use as part of the argument to
make-frame.
The alist describes which parameters were specified in geom, and
gives the values specified for them. Each element looks like
(parameter . value). The possible parameter
values are left, top, width, and height.
For the size parameters, the value must be an integer. The position
parameter names left and top are not totally accurate,
because some values indicate the position of the right or bottom edges
instead. The value possibilities for the position parameters are:
an integer, a list (+ pos), or a list (- pos);
as previously described (see Position Parameters).
Here is an example:
(x-parse-geometry "35x70+0-0")
⇒ ((height . 70) (width . 35)
(top - 0) (left . 0))