The ProForce? Ultra III Female Chest Guard & Sports Top is actually all sized out just like bras, but unfortunately the smallest size is a 36B. So based on that I would venture to guess that the other chest guards that are sized as small, medium, and large; the small would be your size because the 34 part is smaller than the smallest size.
This website has a phone number, you could call them and they could tell you about the product, if it's good for your size.
I'm not too sure but a B cup size is about medium so I would try around there. Also your height counts, if you are on the taller side, try a large, but if you are medium height or small a medium should do.
I'm a bra size 34B, but I don't know how the karate/martial arts chest protection sizing works, could u please tell me what size I should be getting for myself? Need it very soon....