The |largeur filters is applied to an image element (LOGO tag, typographical image, ...) and returns the width calculated for that element once it has been filtered.
The width returned in expressed in pixels.
Example: after having applied the |image_reduire{500} filter to a logo, we can find out the actual with of the final image by writing [(#LOGO_DOCUMENT||image_reduire{500}|largeur)].
See also the |hauteur filter.