crystal3180--
Can u go to a shop and "try on" and 30cm vs. 35cm in a light or neutral tan/brown color? I think that would help solve this dilemma.
for your height, as some have said earlier, either size would work. Oh, sorry; I just read that nearby shops do not have any Birkins to try on.
you mentioned you might need it to carry documents. If that is a key feature for you, then based on that requirement, I would go w/ the 35cm. the 30cm is too small to carry A4 sized documents. (in fact, you might have to fold the papers a very tiny amount in order to fit into a 35cm)
also, if this is to be your 1 and only H bag, I would recommend you get gold hardware--it has less contrast than silver, and it adds a rich touch that I just don't know how else to describe. I prefer silver with cooler colors. Gold is a rich, warm cognac brown, which I think harmonizes better w/ gold than with silver.
good luck and happy contemplating!