I got the x-small. I see that it's noted that the bra runs big, and I think this must be true, as this is the least constricting sports bra I have ever had. I get my regular bras at a real lingerie store; every year I get a discount coupon for my birthday month to spend there. So, I bought a Moving Comfort sports bra, special A/B style, with some padding. I wanted the one with the same type of straps as the Handful, but there was a huge gap in the cleavage area. The salesgirl told me I was just "so petite." Well, maybe in cup size, but my chest itself is not that small. So, I ended up getting the true racer back style and it's very tight. But, the next size up would have been huge.
You probably would take the small if you are a B-C.