I have lots of purses, but I just started using the "smallest" bag made by Timbuktoo for everyday around town and when I ride the train into Cambridge. It fits all my normal stuff, plus if for some reason I needed to go somewhere on my road bike, I can sling it over my shoulder, and slip my light weight Keens in there and take my road shoes off upon arrival. It also fits into the pannier on my Jamis. My textbooks and notebooks are too big for it, though. I think it will fit into my Basil "briefcase."
I also have a really small black leather purse that my husband bought me in Paris. It fits anywhere, but the zipper is breaking. I think I will have it fixed, because it is nice to use when wearing professional clothes.