I actually took a few photos to illustrate the difference in size between the small and medium agendas, because I hadn't been able to find much info on the medium ring. Everyone seems to have the PM!
I've been using my PM agenda for over two years, and while I love it, it's just too small for me. Often I run out of space, and find it awkward to write in. Already I feel like the MM is going to be the perfect size. Particularly as this year I'm working full time while also carrying out my MSc thesis research - plenty to write down!