I think it really depends on you as an individual.
My slimmest [Embarrassingly][sp?] for may years has been 12st7, which at 5ft8in looked pretty good but I was still an 18!!
Although, looking back at those photo's, I look fab and very slim - so I'll be SO happy if I can get back there!
The theory is a dress size a stone though, I believe, although that's never been me.
Good luck hun x