350 m + WOODEN ship before 1800s is INDEED myth, no wood is strong enough to make ship such large.
In fact, above certain size (about 300 ft or 91 m) a wooden ship would be considered structurally unsafe. Only after 1800s and iron used for frame and hull, a ship can finally reach more than that size.
If you don't know already, the 120+ m figure for Chinese (Zheng He/Cheng Ho's) ship is now considered fairytale. The source that mentioned it is a fantasy novel published about 200 years AFTER the voyage. Even modern reconstruction of Zheng He ship (which was formerly attempted to be sail-worthy), in Nanjing is only about 70 m long, and this did not go to sea.
A 120+ m Chinese ship would be about 20.000 ton in displacement, whereas natural limit to the size of a wooden ocean-going ship is about 7.000 tons displacement.
The same problem would happen if you believe any wooden ship (regardless of nation) could reach more than 100 m. The displacement would be too heavy, the ship would break under its own weight (there are several pages on wikipedia claiming ships of such size, such as those of ancient Greece/Roman, but retracing the source cited will make it clear the estimate is usually made more than 100 years after the ship's supposed existence, and the people who made the estimate for the size is not even an engineer or knew the slightest about ancient shipbuilding).
The 350+ m for Javanese you mention is an estimate made by Irawan Djoko Nugroho, who is a philologist not an engineer. Now I do not know how heavy a 350 m wooden ship is, but it is certainly far exceed 7.000 tons displacement as a natural limit for wooden ship.
A 80 m long hull for a 1000-men Javanese Jong may have been make more sense, but there is not enough archaeological data to support this (until now). Admiral Nelson's flagship,
HMS Victory (1758), is only about 57 m in deck length, and 69 m overall. A 80 m long ship would be considered large for a ship existing between 800 AD-1600 AD.