I'm 165cm's tall and Female therefore I would need to get down to 55kgs according to the theory Freddy found. That is the bottom of the BMI for me and not realistic - I'd be a size 8. I just did the wrist thing too...and have just under 6 1/2 inch wrist. I've always thought I was "big boned" but that's because I have always been overweight since I can remember but I am probably more medium. I have been 60kgs since the age of 12 (damn hormones) and 80kgs+ since the age of 16. I was thin and active and a healthy weight range up until the age of about 8 when I got sick (but I don't remember what that was like - it was pre-puberty and health issues).
I took my goal weight from the BMI purely as I don't have a healthy weight I have been or a family member who is in a healthy weight range. I use it as an indication only and a starting point. I have taken the absolute highest figure and may go a few kilos lighter. However the way I see it, if I only get to 70kgs or 80 kgs or even 90 kgs, it's a lot less than I am at the moment - I'm already a lot healthier!. I'm not concentrating on the numbers, more so on what I consume and having an active healthy lifestyle - the smaller, healthier body with then flow...that's the way I look at it