Answer:
I wear a medium.
Explanation:
This one is a bit tricky as both can be said but only one is grammatically correct. You need the article (a). Although when speaking people tend to say both and it works
i.e., I wear size medium or I wear a size medium.
I wear size 6 pants or I wear a size 6 pants.
When writing, I recommend using (a) as this is the correct form. When speaking though, you can be a little freer.
Hope this helps!
