Hi. Well, it is natural to worry about weight gain. However, since you have suffered an eating disorder, it makes we wonder if you are not still wrapped up in the mind set of one with a disorder. That gaining weight would be traumatic for you. Do you still see a therapist for your eating disorder? Are you part of a support group? I think it is really key to stay involved in that.
I think the actual percentage is 7 to 30 % will gain weight with Risperdal. The rest don't. So, maybe you won't. You can stave off weight gain by exercising (the right amount-- not over doing it) and eating a proper diet with the right amount of calories (not overeating). That seems obvious, right? ha. But so often easier said than done.
You sound to be on a low dose which is good. I would try not to let your anxiety about it take hold. Keep to a good diet and exercise regime that is healthy for you. You may gain no weight at all. And if you do begin gaining weight, you and your doctor can determine if this side effect is worth the benefit of the drug.
good luck to you and GREAT job overcoming your eating disorder!! Stay well.