They are not a realistic size any way you slice it. Neither is anything else in the game, now that I mention it.  The only real reason to keep them "large" would be for turreting troops on-deck, but it's debatable whether that would be a bridge too far in the APM department. Maybe I'm nostalgic for the old Age of Empires: Rise of Rome deathmatch naval battles of my youth, when we'd all pick Greeks, Phoenicians, or Minoans and have 100 juggernauts per player duking it out on 'small islands' maps. 

https://gitea.wildfiregames.com/0ad/0ad/issues/8859

What do you mean? this new info is only for one player as everything would be too much. The old one is still there, you may need to check your settings again.

Thank you for figuring this out. 

Instead of making the ships unrealistically smaller, why not make the bodies of water realistically bigger? And then one would zoom out a bit for a proper naval battle.