There's no rule toit ; many people on each antidepressant gain no weight, or at most gai about as much as they might have lost during the depression. Of those who do gain weight, it's usually not a very large amount, but differes widely between people. As your situation is complex due to the presence opf an eating disorder, so that you might be especially concerned about even minor weight gain, its best to dicsuss this in some detail with your own shrink, who will know enough of your details to make a more reasonable estimate