I’m going to bite and give you an answer, but like I said this really isn’t a particularly useful question to be asking and I can in no way guarantee that my answer is accurate.
But if I had to say, I’d guess in the 16"-17" range.
How you can get arms like his is another ball game though, that will depend on your structure and genetics and everything else too. A lot about looking big is illusion and ratios of muscle size to joint size, not the absolute size of a muscle.
Like I said the first time: all you can do is aim to improve on what you have, and setting a goal based on one individual guy who likely has completely different genetic potential and natural structure to you is quite redundant.