system
Given the following post, respond as if you're commenting on Reddit:
human
Is there a theoretical limit to the size of ocean vessels?
gpt
There are many non-theoretical limits:  

Economics. (Cost to build, staff, fuel and turn a profit.)

Port or passage size. (Not simply depth/width of port, but cargo handling limts. It doesn't do your passenger/cargo company much good if it takes weeks or months to load and unload. It also needs to fit under bridges or through canals or breakwaters. Include tugboat size under general port size issues.)

Engine size - (which is both a subset of economics, but has its own limts as well for maximum engine size. But you can theoretically add more smaller engines.)

Material strengths and choices. (This is less important than the above two issues. Google: pykrete to see how to make massive ships out of ice and sawdust.)

And the final hard limit may simply be the curvature of the earth, and any assymetrical gravity loads the ship may experience due to the not-so-uniform structure of Earth and accompanying gravity field.

But the theoretical upper limits for a floating, moving ship are absolutely, staggeringly huge. Look at icebergs for examples. They've documented floating icebergs that are the size of small countries or US states. They're big enough to build several cities on and are measured in dozens or hundreds of miles wide and long, with hundreds of feet of ice above water. If you made a ship out of solid pykrete that size it would likely last for a hundred years.

There's also nothing really stopping someone from building a ship the size of, say, California if you can find a place big enough to float it and it weighs less than the water it displaces.

Moving it around with fossil fuels would probably be too expensive. You might be able to sail it around, or use solar or nuclear for electric drives.

But moving a ship the size of California around would also probably cause, uh, some minor catastrophes like tsunamis, altered ocean currents, altered wind patterns and more.