I have both 30cm and 35cm. For me to decide which one to use really depends on the mood and the needs of the day. However, I tend to use 35cm more when I travel since it holds quite a bit more.
Nice to have you here, this lady has a ton of Hermes knowledge under her belt
As for birkin sizes, I prefer the 30cm, coz I'm only 5"1 and have tried countless time to pose in front of the mirror with a 35, and I just can't pull it off no matter what!