I've encountered such a sentence several times in English exam papers.
What a table! I've never seen such a thing before. It is not half as wide as its length.
I always feel weird when seeing the italicized part. I even can not well translate that part. Is that part of expression good English? How do you understand it?
It's a very awkward construction and I think should be, more correctly, "It's not half as wide as it is long".
But a native speaker is much more likely to say something like:
"What a table! I've never seen such a thing before. It's at least twice as long as it is wide".
not a teacher