[Facebook] Until 2016, the Panama Canal limited ship sizes to 965' length overall and a 106' beam. (The expanded canal accommodates ships of 1200' LOA with a 168' beam.) Merchant ships, and most naval ships (an Iowa-class battleship will just fit), were designed around those parameters. That limited their tonnage as well, to around 50,000 tons displacement. Those ships were known as Panamax and for many decades those dimensions constrained the size of not just most ships, but all ships presumed never to need transit the canal. (Aircraft carriers have not been able to transit the canal since the Midway-class.)I sailed in the US Lines Econ-class vessels, then the largest container ships in the world, on the 'round the world run (90 days) back in the eighties. Those ships were big, and carried just under 5000 TEUs (twenty-foot equivalent length) in containers. They were a pig in the wind, because of their high sail area created by their deck cargo of containers, just as aircraft carriers, car carriers, and roll-on roll-off carriers are. At sea wind can be accounted for in the course one steers and by speed of the vessel, just as sailboats do, although such ships, when the weather got really bad and they hove-to in order to ride it out, were a hot mess. Their sail area ensured they'd end up with the wind on their quarter, which isn't entirely a bad thing, though they will then roll, and roll heavily. Their sail area was of yet greater concern when tied up alongside a dock or pier, when winds got up over 30 kts. All the ships lines could not hold them alongside and anywhere from three to five tugs might be called out to hold them alongside, depending upon the size of tugs available.
A brief word about harbor tugs. Harbor tugs commonly carry around 1000hp engines, though there are many still around with far less horsepower still earning a living. The Japanese, who have some of the finest tugs in the world, ran tugs up to 3000hp, and a couple of them would push the largest ships in the world alongside with no problem. The newer tugs in Japan and Northern Europe may produce 5000hp. That's a lot considering the containerships already discussed possessed around 35,000hp.