Don't base your target on dress sizes as there is no industry standard and they vary between brands. What is a 10/12 anyway? At 5' 9" and under 10 stone my wardrobe covers size 10 to size 16, yet I couldn't fit into a size 22 jacket in M&S?!?
Concentrate on being in the healthy range for your BMI. If you need to lose weight, as opposed to want to lose it, a healthy rate of weight loss is up to 2lb/1kg a week.
Remember you can't spot-reduce fat, so may not lose it from your face, or not initially, at least.
Basal metabolic rate (BMR) is what you need to function at the most basic level; that's the minimum number of calories you need each day. The average daily rate to maintain your weight is around 2000 calories for a woman. Your daily intake should be between the 2 figures to lose weight. There are online calculators to work this out as it's all down to the individual. Being in a calorie deficit isn't just about cutting down the amount of calories you eat, it's also taking into account what you burn off through exercise. You could do an exercise class that burns off 300 calories and reduce your food intake by 200. Don't cut your food intake by 500 calories, then do loads of exercise and that takes you below your BMR; it will seriously impact your health.