BMI is a poor judge of being overweight etc, it doesn't take into account any muscle you might have which is heavier then fat - personally though I would say size 12 is on the upper side. Generally the taller you are, the 'larger' the dress size you can be, without it looking out of proportion.
Pinch the skin and fat just above your pelvic bone, how much is there - that's a good test of how overweight you are - the less the better.