McGrath: The largest sea going junks, built in the 15th century, way have been up to 120 metres long . Not that the 15th century is antiquity, but the construction would have been entirely in wood.

As far as I know, this is an estimate based on extrapolating from a single sternpost that was found. The "may have been" 120m size is the upper end of a range of "guesstimates" and the consensus among maritime architects is that the actual size would have been a fair bit smaller than this. It's still a good bit smaller than the supposed length of the ark.

We're back to the upper limits of the strength of wood again, which is simply a physical limitiation. There is AFAIK no reliably recorded case of a vessel constructed wholly from wood and which is longer than 100m being seaworthy. The few historically verified wooden vessels that we know of which were around the 100m mark were at best marginally seaworthy.