One use of such a phrase in English could be to do with standard clothes sizes, i.e. " He is a size 10" - but it would be good to have a clearer explanation of why the feminine form of "eine" is required here, when that meaning is not indicated.
I assume the latter. It's an odd and random, but grammatical correct sentence. The only explanation that fits translation and original sentence would be a rating of his appearance, but I'm really just guessing here.