Welcome to the PF!!! That is such a tough question which many of us struggle with! I don't have a birkin YET, but I can tell you that I have tried on a 30, a 32cm HAC and a 35cm. From what I have experienced, and from what I have heard from many of our members here, the 32cm HAC is the perfect size IF IF IF you are ok with a handheld bag. Because of its shorter handles, you have to be comfortable with that style. Also, the 32cm HAC is deep, so it can be a pro or a con depending on your needs!
With the 35cm, I loved the size when I tried it on, and the only concern I have is the weight. Some say it is no big deal (hell, I am used to schlepping around a Paddy and a MJ Blake) while others say it is very cumbersome in a 35cm especially in leathers such as Fjord, Togo and Clemence. So really, the best is if you can go try it on for size to see which size/combo works best for you! But if you are ok with a little weight, then maybe the 35cm is the way to go!
Good luck with your decision!!!