Do you have to be a specific weight to be diagnosed as anorexic?

Anorexia and all other eating problems are disorders of the mental variety. They have physical consequences, but the actual problem is in your mind. So, if you've got the mental symptoms of anorexia, then just because you happen to barely be in the "healthy" range of BMI, doesn't magically make you not anorexic anymore. Also keep in mind that the BMI scale is just a general tool and not necessarily accurate for every person.