Mira, to be really honest, the choice of size is not an easy one. I still find myself flippant at times when presented with a 35cm before me. But I know a 30cm suits me better as a 35cm can very overwelming.
I love big bags. That's why one of my earliest H bags is a 42cm JPG shoulder birkin. The downside, is that it's very heavy. But it's to be expected. Birkins are 100% leather. So this didn't come as a rude shock or anything.
The sizes I would choose for all other brand bags like Tods, I would not choose for Hermes bags. Hermes bags are heavy and it's a factor that you cannot ignore. If you think you can deal with it, great. If you think you can't, then re-think.
I am 5'4" and proportionately, 30cm is the size for me. But I find myself over-stuffing my 30cm birkins to the brim, and that attracted my manager to recommend me the 35cm. I tried one, and had a hard time deciding whether or not to give it another shot. At first, I said "Yes" to the 35cm. But the decision made me so uncomfortable. And days after that, I went back to her and said, "No. 30cm suits me just fine"
My manager told me before that the 35cm has more oomph factor. I have to agree with her. It's just not for me.
And you know what, the sad truth is, you really need to own one to know if it's for you or not. No amount of rationalising is good enough, until you actually own one and try it for yourself.
All the best.