No worries! I have one 40 cm. It is BIG (and I'm relatively tall at 5'8") but it is carryable for me, and definitely an awesome travel bag, as xiaoxiao says. For an everyday bag, I stick to 35 cm, and with the current emphasis on mini-bags, I've even been considering trying a 30.
ETA: Oh, phooey, I just read your post properly, and you are talking about a 42 cm SHOULDER BIRKIN! Like the JPG, or its second iteration. You are completely right and I was totally wrong, the shoulder Birkin is 42 cm. I'm so sorry. (READ next time, Mindi!)
Ahem. I have two shoulder Birkins. I love them as they are a bit unusual. They are heavy and big, so be sure the weight and length don't overwhelm you.
And, again, so sorry for my error.
Duh.