I think this statement is much akin to if a creationist made the challenge "Put a reptile into a laboratory, and induce it to evolve into a mammal". Evolution does not happen like that, in such a short time frame.
But of course I wasn't challenging the perspective that greater oxygen levels could cause populations to adapt by increasing in size. I was challenging the YEC perspective that exposure to oxygen causes normal individuals to grow to immense size - no adaptation needed.
It's ridiculous, of course - oxygen doesn't have that kind of effect on animals. It's not a magic life-gas. It's simply an avaliable reactive gas that animals can take advantage of to metabolize sugars to a greater degree of efficiency.
Now, maybe it could be that, in an oxygen-rich world, animals would adapt by becoming large. I don't know why, but maybe they would. However it would be slow, and take geologic time. Hardly a YEC perspective, no?