I have no problem breathing in the x-small at all. I mean, you can feel the compression when you first start wearing it, but I can work out and take full breaths in it. But that might also be due to having worn them for so long. To be honest, I don't know my measurements at all. If I were to even guess at my cup size, it's probably a small B or so. And I have too much length. If you're a D, then I'm not sure how much length you'd end up with. But a small C and below I don't think you should have any sort of length problem. I can almost double up the binder, as in roll it over the top of its self, completely before I get to the bottom on the chest.
If you're scared about breathing, then try the small. They're pretty good about exchanging in the end. Most people say to go with the measurement the site gives. From the beginning I've always gotten an x-small, because that's usually what I wear in men's clothing for the clothing to fit nicely, not somewhat loose.
