I'd go for about a stone too. Two stone is very achievable, but can seem like a bigger target therefore you're more likely to be disheartened if you don't get there! You'll feel happier if you can make a stone, which I'm sure you will!
SW is better achieved ay you setting short term goals, with the target somewhere in the distance. Achievable, measureable, timely targets are far better. i feel that this approach has kept me on the ball and most of all motivated.
Don't set your self up for failure hun by giving yourself large targets to achieve. As Phil says, go for the smaller ones, achieve them, feel fantastic and move on to the next goal. Have you seen the valentines challange thread, you could join in with that which will take you to the middle of Feb and then you would only have about 4 weeks left on your pre pay for another little challenge.