I am 5'6'' (well actually 5'5.75''

) and currently I am about 13.5 st and a size 18
Some stats from previous times though (bust/waist/hips in inches and dress size in brackets):
BMI 32: 43(18), 37(20), 45(18)
BMI 31: 43(18), 36(18), 44.5(18)
BMI 30: 43(18), 35.5 (18), 43.5 (18)
BMI 29: 42(18), 35 (18), 42.5 (16)
BMI 28: 40.5 (16), 34 (16), 41.5 (16)
BMI 27: 40 (16), 33 (16), 41 (14)
BMI 26:
BMI 25:
BMI 24:
BMI 23:
BMI 22.3: 36 (12), 27 (10), 38 (12)
Basically, I know I am obese when I am in a sz 18, overweight in a sz 16, upper healthy/lower overweight in a sz 14, healthy in a sz 12....and I have never really been a proper sz 10 but I would guess I would need to be about BMI 21 for that.