I sympathise with you Islonchick. I'm in exactly the same situation. I also met my boyfriend a year ago, I was an athletic 10 stones 5 pounds. Obviously through getting comfortable and eating out too much, my weight rose to 12 stones 12 pounds. I'm about a size 14/16 and 5'6 tall.
Like you, I've been dieting for a week, (started juicing) and I've also lost 7 pounds. As for whether people think 12st 8lbs is large for your height, well it's hard to know without seeing you, you could be muscular or large boned, which would naturally make you heavier than someone of the same height, who had finer bone structure and smaller muscles.
In the UK a size 16 is the average size for women, but it really comes down to how you feel about yourself, not what others think. I feel I am too big now and none of my size 10 clothes fit me anymore, so I know for my own self esteem, fitness and health, I need to shed at least 2 stones.
I wish you success with your weight loss.