window-pixel-width

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

(window-pixel-width &optional WINDOW)

Return the width of window WINDOW in pixels.
WINDOW must be a valid window and defaults to the selected one.

The return value includes the fringes and margins of WINDOW as well as
any vertical dividers or scroll bars belonging to WINDOW. If WINDOW is
an internal window, its pixel width is the width of the screen areas
spanned by its children.