[PIF]

Ship building was already in decline - the Greek Syracusia is believed to have been 180 feet long. No comparable ship like it was built until the 18th century

[pierrem15]

The Diadochi built some huge ships, but I'm not sure if any were actually put to sea or were intended only for harbor defense.

Ptolemy IV Philopator of Egypt built an oared catamaran so large each oar required 40 rowers. But I don't think it ever left the harbor. Similarly, Caligula's barges were immense.