That's a bit of a length of a piece of string question, Sean. With trains the forces (mainly friction and gradient) are fairly predictable, but with ships (incl. barges, rigs, etc) wind and currents are a large factor alongside simple inertia of course. Plus trains can only go back and forth on the rails, ships can move in any direction - until they hit something, like land.
Boats move fairly easily through the water (even I can get a boat weighing a few tons to move quite easily), but if the wind blows I can quickly find it getting away from me. So the displacement of the ship to be 'tugged' is not actually a big factor.
Towing companies and ports have ways to decide what tugs are needed for a job. I found a formula for required bollard pull in the Port Designer's Handbook, and it contains factors for wind, gust, current, wave and a safety margin. The factors are calculated on the areas of ship exposed to wind or water - but not the displacement of the ship. (Generally a bigger ship will displace more, but for example a loaded oil tanker will be mostly underwater with little windage relatively speaking, whereas a container ship of the same sort of size stacked high may be quite the opposite.) So if it's a calm day at neap tides one tug might do the job; if it's blowing hard at springs you might need several.