I have the medium and chose it for its versatility.
I'm 5'8 and bigger bags tend to look better on me. However, I got a small sac de jour, so maybe that isn't always true. I felt that the small antigona looked too small on me and I was not liking the strap (since it isn't long enough to carry it with the strap without the top handles getting in the way). In my opinion, the medium comes with a longer, more practical strap. I use it for travelling and tend to wear it behind my back with the strap. If I am carrying it during the day to run errands, I don't use the strap at all and just use the top handles!