The film itself is 35 mm wide (and several meters long).
The image size on 35 mm film is 24 mm wide (between the perforations) and 36 mm long in normal film photography cameras.
Hence: the common "full-frame" image size of 24×36 mm, used for d*****l cameras.
Old half-frame cameras like the Olympus Pen (1960's) use 24 x 18 mm on 135 film, thus making 72 images on a 135 film.
See also: http://en.wikipedia.org/wiki/135_film