Why not "Je suis" or "J'ai" instead of "Je fais"?
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.
thank you ; extra lingot.
The size could also be a length, but the taille is the waist: My waist is fifty should be correct?
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.
Or: I have a waist of fifty
'I am a fifty size' must be also accepted.
When would you use that word order?
Sometimes I heard that word order. I don't know if it is acceptable.
No, the number should follow the word size if you mean a person or item of clothing.
Thank you for this information.