You do realize that even I, who am less than 5 feet tall, would be severely under-weight if I were 90 pounds, right? Sure you can have a preference for skinny girls, but it's dangerous to put a number on that.
Being 90 pounds is not fat, it's basically underweight. I'm not sure what's your definition of being "chubby", but not wanting to date a girl because she's "chubby" at the weight of 90 pounds is not right. It shows that you date girls only by their appearance.