Without getting out the DSM-5 to give you the exact criteria, I don't want to say too much, officially. What I can tell you it that it depends what disorder. Let me give you an example:
I have had an on-and-off issue with ED's. In my early 20's I thought I was being pretty clever in terms of my bulimia because I'd binge, purge, and then eat a moderate amount of food so I wouldn't be oddly thin. (The mind plays some interesting tricks with this one - I started strictly to be thin, then it would pop up years later as a means of coping with a stressor rather than just staying thin. The motivation changed).
Years later I thought getting chubby was a good thing, in a way because I had finally stopped purging altogether.... but still was binge eating in response to stress. That's when I was diagnosed with Binge Eating Disorder.

No, you don't have to be underweight to have an ED. In fact, many bulimics are normal to above average weight.

you absolutely do not have to be thin. ED comes in a very wide variety. You can be addicted to food and be over-weight, you can also be anorexic and over weight because your body stores fat as it goes into starvation mode. ED is classified by a disorder that is related to eating. If you do not eat in a "normal" fashion (you binge, purge, ration, etc) then you have an eating disorder.

Thanks

I'm overweight due to medications that I was on and I have an ED. I can go up to 22 days without eating and then eat tiny amounts and not lose weight but the ED is still there.

This is the biggest trick the ED will play on you, telling you you weigh too much to have an ED (I still argue this thought with myself daily). You can 100% have an ED at ANY weight and the majority of those with EDs are of normal weight or overweight. EDs are a disease of the mind and focus around the thoughts and behaviors associated with food. Has this helped you sort things out?

Yes LP. Thanks everyone

ED is the action that follows an underlining secret or issue that we have not faced up to yet. It does not matter what weight we are. Therefore, we choose to avoid that issue by focusing our mind and energy on food and our body image. Size does not matter. Good luck finding your issue!

I know a young woman that didn't seek treatment for her ED because she didn't think she was sick enough (thin enough). EDs come in all shapes and sizes. If you suspect you have an eating disorder you probably do. Seeking help to determine what's going on inside you is a great first step.