I'm 5'5", 123lb, 32DDD chest, I ordered a size 4 which is what I wear in everything Oh Polly makes (or a size small if that's the way it's sized).
This is a really cute dress, very sparkly which I love, it shines in any light and reflects beautifully. I like that the straps have some shape to them, and I like that it's not too low cut because I have a larger chest.
It's a little see through but you almost wouldn't notice it if you weren't looking for it. Just put some nude undergarments on and you'll be fine.
It's a little boxy looking, but it's still very flattering. My one gripe is that it looks a little "cheap." It's well made but the big mirrored rhinestones just give it kind of a cheap feel and look. But it just shines so pretty. It's a versatile party dress, concert dress, bachelorette party dress etc. I wouldn't wear it anywhere that calls for cocktail or black tie because it looks a little too cheap for that but it's fun for the other things I mentioned.
Disclaimer that I initially returned this dress because I felt like the rhinestones made it too cheap. I also showed 3 men this dress and another dress (the assymetric body con mini dress in gold) and all 3 men said the other dress was nicer and sexier, but I regretted the return and re-purchased it.