There's a huge variation in ships.
It's not JUST about size, it's about design...angles matter in a big way.
And so too does movement through the water. As long as the boat is moving forward, seasickness is actually less likely. It becomes more of an issue if the boat is not moving, and simply bobbing up and down. The direction that the boat is moving in relation TO the waves is also an issue.
You also have to take into account stabilizers. Most large passenger ships today have them.
Then, you also have to take into account WHERE on the ship you are. Different parts of the ship will experience these things differently.
If you tend to get seasick, cabin location is really important. It's a question of engineering, really. The lower and more central you are in a ship, the less roll and sway you will feel. Even if you choose a balconied stateroom, choose the lowest level and the most midship one you can find. The higher decks and cabins at the very front (forward) or back (aft) of the ship will rock and roll the most.SOURCE
The average cruise ship has a tonnage of 110,000 and length of 952 feet.
First, let's define "moderately inclined" to seasickness and "not unusually rough water."
Let's say we've got a World Meteorological Organization sea state code of 3. That would be 1 ft 8 in. to 4 ft 1 in. So 4-foot waves would not be felt on a ship this large. All of the other factors have to be taken into account, but... 4-foot waves are NOTHING to a boat this large. In my experience, 7 footers aren't much of a problem most of the time either. When it gets to 10-15 ft, you'll feel it, but again, this really depends on a) if the ship is moving and how it's moving in relation to the waves b) where you are on the ship and c) stabilizers (in some cases the stabilizers made it more of a slow roll, which can actually make things worse in some parts of the ship).
I would say that I am moderately inclined myself, but this not only depends on all those factors, but also what and how much I have eaten, and how hydrated I am to begin with (something that should be taken care of days before).
So the answer is It depends on a whole host of factors, and how you define moderately inclined, and what "not unusually rough water" means EXACTLY. There is NO MAX size for this.
“There’s no way to prevent motion sickness 100 percent,” says Dr. Art Diskin, Royal Caribbean Cruises Ltd. Global Chief Medical Officer. SOURCE