Dysmorphia refers to difficulties in judging shape or form. People with dysmorphia may be unable to tell a circle from a square, or a triangle from a cube, even though they can see normally. Apperceptive agnosics often suffer from dysmorphia in this sense of the term.
The term is also used more broadly in reference to body dysmorphias--that is, misjudgements about the size and shape of one's body. Anorexics often suffer from body dysmorphic disorder, and think they're fat when they're really emaciated; people with muscle dysmorphia think they're scrawny and weak when they're musclebound. I suppose the term could be used to refer to people who are blind to their own obesity, but I've never heard it used in that sense.