Bonjour You can say : Quelle est ta taille? (if you know the person like your best friend) or Quelle est votre taille? (if the person is a shop assistant for example) In French you have to becareful who you are talking to...

you can say in shoes for example ,,,,, quelle est votre pointure? in clothes ,,,,, qelle est votre taille? and i also agree with munkelmah.

"Tu mesures combien?"