I wear a ladies size Rolex, I think it is 26mm. Although I am a large person (5'7") and have a larger wrist, I do not like the way a big watch looks on any woman. I think it looks like she is borrowing a man's watch.
I also wear a ladies datejust that is 26mm. However, on me, I loved the BB in the medium size. I think I felt as though the small was too close to the datejust, but in any event, I don't think you can go wrong with either.
I tried on the small and medium again and think the medium fits the style of the watch better. When I tried on the small again I was a little underwhelmed. So like you leaning towards the medium automatic.