As for my 2 cents, I have tried on both the 35 and the 30 and to be honest, the perfect size for me is the 30 because I am 5'1" and feel that the 35 is just too big. If Hermes would just make the standard birkin in a 32cm, it would be the ultmiate perfect size.
I love the reference guide! This is absolutely brilliant and very handy! I've tried on a JPG, 35cm, 30cm, and 28cm HAC before. I have to say that I prefer JPG, 30cm, and 28cm HAC the most ... I'm 5 ft 8 in (add an inch or two to the height when I'm over 130lbs) and while I can carry off 35cm, I found the smaller size to look more proportionate to me as a purse. 35cm definitely looks more like a tote to me. JPG works on me because the East-West shape takes away from its overall size.