window-text-pixel-size

window-text-pixel-size is a built-in function in `C source code'.

(window-text-pixel-size &optional WINDOW FROM TO X-LIMIT Y-LIMIT MODE-AND-HEADER-LINE)

Return the size of the text of WINDOW's buffer in pixels.
WINDOW must be a live window and defaults to the selected one. The
return value is a cons of the maximum pixel-width of any text line and
the maximum pixel-height of all text lines.

The optional argument FROM, if non-nil, specifies the first text
position and defaults to the minimum accessible position of the buffer.
If FROM is t, use the minimum accessible position that is not a newline
character. TO, if non-nil, specifies the last text position and
defaults to the maximum accessible position of the buffer. If TO is t,
use the maximum accessible position that is not a newline character.

The optional argument X-LIMIT, if non-nil, specifies the maximum text
width that can be returned. X-LIMIT nil or omitted, means to use the
pixel-width of WINDOW's body; use this if you do not intend to change
the width of WINDOW. Use the maximum width WINDOW may assume if you
intend to change WINDOW's width. In any case, text whose x-coordinate
is beyond X-LIMIT is ignored. Since calculating the width of long lines
can take some time, it's always a good idea to make this argument as
small as possible; in particular, if the buffer contains long lines that
shall be truncated anyway.

The optional argument Y-LIMIT, if non-nil, specifies the maximum text
height that can be returned. Text lines whose y-coordinate is beyond
Y-LIMIT are ignored. Since calculating the text height of a large
buffer can take some time, it makes sense to specify this argument if
the size of the buffer is unknown.

Optional argument MODE-AND-HEADER-LINE nil or omitted means do not
include the height of the mode- or header-line of WINDOW in the return
value. If it is either the symbol `mode-line' or `header-line', include
only the height of that line, if present, in the return value. If t,
include the height of both, if present, in the return value.