Furthermore, in order to obtain the same level of torque in a single-cylinder that a multi-cylinder engine creates you'd have to account for the power generated per cylinder.
In the Wartsila RTA96C you have HP up to 109,000 (rounded) and torque up to 5,600,000ft-lbs [1]. These #s are for all cylinders. Assuming it is the 14-cylinder variant generating this power and you want to reduce this down to a single-cylinder package then you can easily deduce that the displacement would be around 25,480 liters (mentioned elsewhere in this thread).
Assuming a stroke to liter ratio of 1.374mm/liter (2500mm stroke for each 1,820 liter cylinder) you'd end up with a 35,009mm stroke for a single-cylinder engine vs. a 14-cylinder engine. Since there are 25.4mm in an inch that's a 98-inch stroke for 14-cylinders vs. a 1,378-inch stroke for a single-cylinder or an 8-foot stroke vs. a 114-ft stroke.
Finally, since you'll need a piston many times larger than the current setup for a single-cylinder vs. a 14-cylinder engine then you can imagine how much mass you have moving which can do some serious damage if it becomes unbalanced. Also you'd have an engine that instead of being long and relatively low is now very short and extremely tall (not something you want in a ship where capsizing is a very real concern).
[1] http://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA96...