A jacket is a thing and a size is a measurement and a jacket can not be a measurement.
Why do we say "This jacket is the wrong size" which sounds like "This jacket is a measurement"?
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That is how the word "size", and similar words like "age" or "price", works
So you can say that it's the wrong, or right size. Or you can ask "What size is it?"
And you can say "What age is it?" or "It's the right price" and so on. I don't know if this class of words has a name, but you should be able to think of some more that can be used the same way.