Source: I've actually been employed to work on this very topic before.
Indeed if a floating structure is large compared to prevailing wavelengths it won't be rocked much by it, and there isn't much wave energy with wavelengths of many hundreds of meters. So rocking won't be your problem with a monolith of this size.
One problem with with a monolith is a not particularly intuitive one (that we usually don't think about because most of what we engineer is small compared to ocean wavelengths); wave reflection reaction forces. Objects that are big relative to a wave dont transmit, but reflect. The momentum imparted by a reflecting wave scales as the cube of the amplitude. For long wavelength high amplitude storm waves, the forces pushing you are really crazy, and no conceivable mooring system short of making this thing an island will keep it in place. Of course you could argue that's alright; have it be pushed around a bit by big storms, and have a form of dynamic positioning that keeps it where you want to be in the long term; but if this is to be a city where you'd want about 10x more space per person than a typical boat, and this thing is more than 10x less hydrodynamic than a typical boat (very generous), just staying in the same place against currents and winds is probably going to become quite an expensive affair. Putting it in a self-stabilizing location like an ocean gyre has been proposed; but there are many factors that dictate where you might want to place this, and this additional constraint isn't particularly welcome. Also if this gyre changes and your stability argument fails, there don't exist sufficient tugboats in the world to correct your error.
The simplest solution to all this: just go below the waves. Surface wave effects decay exponentially with depth. The static forces of going a few tens of meter below the water are an absolute walk in the park, compared to designing for the almost unbounded worse case scenarios on the surface. Also surface currents and obviously wind are much less of a concern here; though you will be displacing a lot more water per unit of usable volume, so the economics of dynamic positioning is still something you should keep an eye on. Also there are psychological issues; it just seems less appealing to live under water like that. You could imagine a system that sits on the surface most of the time when conditions are reasonable, but has the ability to flee under the waves when required though. Though the deck of a submarine typically also isn't that appealing of a hangout, so there are definitely open questions there.
If you want to be permanently on the surface, a loosely coupled fleet of elongated ship like shapes is a reasonable approach. They can orient their long dimension into the prevailing wave direction (typically there is such a thing; in cross-seas you are on your own w.r.t. comfort, but some geographies make them super rare) to get good stability most of the time, but are sufficiently hydrodynamic for dynamic positioning to be feasible. Probably you want a system of bridges/linkages between them; and maybe those could contribute some to stability in reasonable weather; but most likely you'd want to make that system retractable to prevent damage in worst case scenarios. If a single ship module was 1km long or so, it'd be so stable along the front/back axis that connecting them head to tail into multi-km structures would be relatively simple. (100-200m or so would be a very different story; that's right smack in the middle of the predominant wavelengths of big storms.) Longer structures have worse bending moments to contend with obviously; but that's a solvable engineering problem. If your ship gets longer, you need to make it higher to get good bending strength; and then also wider for roll stability reasons. So if the size of your module is say 1000x80x80 meters that's double the length and roughly 8x the volume of the largest ship ever built; big but not unthinkable, and still significantly modular compared to a totality of multiple square kilometers of area.
As alluded to above: linking floating objects together on the open ocean to withstand storm forces is an unsolved problem. Again, the maximum forces involved are insane. Even welding two halves of a boat together so it doesn't break in half in a storm isn't easy, so you can imagine how it is when you try and make it more complicated. Such linkages both need the range of motion to allow a multi-tens-of-meter wave to pass, and the ability to transmit insane forces. I am not saying impossible mind you, but how to actually engineer that system is to my knowledge an unsolved problem. Probably something involving huge hydraulic cylinders, with a very long stroke, but also the ability to output insane forces, would be required. I've never gotten deep enough into the details to figure out if its really feasible. If we are talking about a linkage this beefy, making it easily detachable sounds like a challenge if not impossible, so you really need to engineer it for worst case conditions.