Great question, bro!
In this industry, the girl's dress size is used to convey a body type ranging from skinny, slim, curvy or fat. A skinny girl will be a 4 - 6, slim girl would be 6. Practically anything above 6, the shop or girl is trying to convey that they are not the slim type but more voluptuous, a fuller bodied woman. That's how it works with shop/girl descriptions, but it's not a clear reflection of how women's clothing sizes work.
For one thing, there is no size 5 or 7. The sizes go up in even numbers, 4, 6, 8, 10, etc. Second, a slim girl with a bigger frame (eg. Taller, wider hips, etc.) usually would have to wear size 8 clothing, but they would still look very slim.
Anyway, as far as the intended effect of this description goes in punting, if by seeing "size 7" in a girl's description gives you pause because you favour slim girls, then they've done their job. That was the message that they were trying to convey to you.
All the above was based on the assumption of 100% honesty from the service provider, of course!