I spent a summer in 1964 as a deckhand on the Arthur B Homer, the near sistership to the Edmund Fitzgerald. The Homer was a big ship as Great Lakes oreboats go at 731 feet. The old "canallers" were still around but about the only ones still in service had been streched to at least 400 feet. Anything smaller was just not economical to run, especially since many of them were still coal burners at a time when most shipping had changed over to oil. So, it depends on what era you want to model. If it's any time after the late 50's, the boat should be at least 400 feet. Before that, the original size boat shown in the link was common although the dedicated ore boats that made run from Cleveland and Detroit up to Duluth still averaged about 400 feet even in the 30's. The iron ore docks were something to see in person and would make a great model but I suspect it would be almost a lifetime job to get it all done.
As Ken L. wrote, you will have to do a lot of selective compression. If I remember correctly, the Cleveland Cliffs ore docks in Cleveland were just a little under a mile long and could unload six boats at a time. The shipping season on the Lakes is short and ore companies wanted each boat to make as many runs as possible so loading and unloading went on 24 hours a day and then you went back in transit. I think the longest I spent on terra firma those three months was about 18 hours because one of our propeller shafts needed repair. Average turnaround time was six hours.
