We tend to give the shorter dimension first. This is true of both film and paper.
For example, a 35mm frame is 24mm x 36mm, even though 24mm is the height, not the width.
The exception is 6cm film. Since there are so many formats using that film, the 6 is usually given first (6x4.5, 6x6, 6x7, 6x9, 6x17, etc).