I hope to be as openminded in the answer I give you, but I can only try.
First of all, I think Hayley gave a poignant answer, especially about the way you tell yourself that you can't lose weight being a blocking mechanism in your mentality of actually losing weight.
From my point of view, if you want to lose weight, it has to be for the right reason. Please don't think that losing weight will bring you instant attention from the boys. Striving to be more attractive in a common societal frame will bring you the kind of attention or oogling from the opposite sex that you may feel objectified by, but it will be just that - attention to the way you look, not who you are inside.
However, from the evolutionary point of view, men are attracted to women that display healthy breeding traits and physical attributes - as in the hip to waist ratio. That is mostly what makes men sexually attracted to women. If you are looking for a man to have a strictly sexual relationship, with the possibility of a long term courtship, then yes, you should think about becoming more physically fit, if that is what you are looking for out of a relationship.
Now, as for what you are supposed to be, or what size you should be, well that depends on many factors. Some of which include your height, bone density and frame size, genetic factors, etc... You should go to a respectable physician that will give you sound advice and not try to make you feel bad about being overweight - believe or not, there are a lot of doctors out there that are prejudiced and emotionally or mentally demeaning to people of larger size. It will be a benefit to you to find someone to council you through weight loss and the emotional side effects that have taken a toll on your self-esteem as a young woman.
Overall, your question about how big do you have to be to qualify as repulsive urges me to remind you that there are a lot more attributes that make a person, whether male or female, repulsive. An exorbitant amount of faults make someone repulsive, regardless of weight, size, or appearance of any nature.
Should you strive to be a thinner version of yourself, just please remember that version will still be you, only with less fluff and possibly a bit healthier. You will still have the apprehension that inhibits you from knowing that you deserve someone that treats you with respect and wants you for you, seriously what's inside. And please don't take this the wrong way, I only say this because you present yourself this way.
Take care, and remember that your attitude about yourself is THE most important aspect to your attractiveness.