http://forum.purseblog.com/hermes/do-you-prefer-size-30-or-35-birkin-7266.html
this thread has a lot of information on 30cm vs 35cm. i personally like 30cm better and thinking about moving down to 25cm also. you can totally go casual with 30cm by wing batting it.
Thanks Honhon. Useful link indeed. Maybe I need to play around with my 30 a bit more. Maybe the color, I feel the bag is a bit formal.