An arcology doesn't require to be physically linked objects. That means it can be a group or groups of ships, sometimes connected to each other. As each boat has their own propulsion, you're basically limited by the slowest in speed and turning, assuming no assistance is given. The size of free floating conglomerates of ships could be defined as limitless. You probably would hit some unexpected maximum at a certain point, like the propulsion would heat up the water significantly or individual transport between boats would become an impractical mess, but it would be at a huge size.
If you want it to be a connected whole it needs to have flexibility in mind. A piece of A4 paper is strong. A piece of paper identical to A4, except it covers a square kilometer would rip itself apart quite quickly with a stray wind. The larger the size, the higher relative stresses become. A more nautical example are boats themselves. When moored at a marina or against other boats there are no rigid fixings. The boats are allowed to move up, down and a bit sideways/font/back. Otherwise incremental damages would break the ship, and rigid supports would easily break.
On the ocean you'll have waves and wind aplenty, making it imperative to have flexible supports. These flexible supports would make each boat basically independent of the rest, but maximise the range they can float off. It is likely you can still make some transportation on these flexible supports (ziplines as an example). It would still be a good idea to be able to unfasten the ships from each other.
You would end up with a lot of boats. Again, with each of the boats having propulsion moving shouldn't be a problem. Turning however is mostly determined by the speed of the ships on the outside. They have to move much faster than boats in the middle, so turning ratio's are reduced. It would be more logical that the boats don't have a direction they would face and the propulsion would pick a different direction, or that each boat individually can turn on the spot. That way not the whole flotilla has to turn for a new direction.