I'd say they're TTS, but since they're jersey they have a lot of give. I'm a 2-4, depending on brand, and I have two pieces of RP in a small (the lovely Lilykind picked them up for me at a sample sale) and they fit fine. I have another dress in an XS and it's perfect (it's a looser-fitting style).
I had to size up in that one, but that's because I have fat arms and a large chest. I was definitely an M. If I had normal arms and chest, I would've worn an S.
kittenheels has this dress though, so hopefully she'll see the thread!
First of all - I love this dress! I am a 4-6 and a 34 D...I bought the medium because I found the v-neck was a bit low with the S. I am also 5'7 and the S was a little short - I wear this dress to the office, so I wanted it to hit around the knee. It's an awesome dress - so incredibly comfy! I own it in that exact color too! I hope you love it!