Even the tiny one does take in a lot of stuff, but if you frequently need a huge amount of things with you, I'd go for the Medium. I have an XS and a L but the Medium is still non existant so will have to make up my mind soon and get one. This design is SO gorgeous, like a Mary Poppins bag - but that's LP for you; they fit a whole heap of stuff even though they may look small.
You might find some answers on the "What's inside your Longchamp?" thread, here's my 5 cents on the XS size re the amount of things you can carry in each one!