I'm 5'5" too (or possibly 5'6" I'm not sure) and I think I remember seeing somewhere that 55 - 65kg is a 'good' weight for that height.
Personally, I think anything under 52kg is too skinny. And... hmmm... anything over 68kg I think I would want to start losing some weight. I'm not really sure if there's an exact weight that I would call fat - perhaps getting beyond 70kg? It really does depend on the woman and her body structure too. I guess perhaps I am being a little hard on this, cause I can speak from experience since I'm this height myself. I fall into the 'good' weight area.