以下兩句那一句才是正確? 1) May I ask what size you wear? 2) May I ask what size do you wear?

(2)May I ask what size (do) you wear ?
(do)=v.aux. makes interrgative question and expresses Emphasis (did pt---done pp---doing pres.p.)
Formally add the (do) for good manner.
(1)May I ask what size you wear ?
without the (do) is informal conversational talk with customers, to be acquainted,familiar with.The oral conversation is enough in Doing Business.

1) May I ask what size you wear? 是正確的

假如講文法正確或要二選一的話：2) May I ask what size do you wear?
但是假如是真正的英語談話：May I ask what size are you?不過小心這是不太禮儀和較敏感的問題。