pm mm and gm stand for sizes, correct, with pm being the smallest and gm being the biggest. BUT they do not have specific measurements to them, for example the croissant mm is not the same size as the seleya mm, it's just that both are the medium version of the bag.
yay thanks raspberry! i was going to type something along that line but my knowledge of french is close to nothing so I saved myself the embarassment, haha.