This is idiomatic to the language. Je suis and j'ai do not make any sense to a French speaking person. This is just like "I make" a size fifty wouldn't make sense to an English speaking person.
Je fais une taille x is idiomatic to mean I am a size x. If you mean your waist specifically you can say tour de taille to mean waist circumference or if you need a chest measurement, it's tour de poitrine.
Je vais mesurer mon tour de taille.