I am aiming for the same weight and I'm 5 foot 7....I nearly got this size before and I was nicely between 12 and 14 as I have wide hips...was a lovely size, and realistic too. How are your hips - and your bum - I have big on both counts so 12-14 is a good aim for me. any lower and i'd have to deprive myself of anything remotely fun which is unrealistic.