A glance at this wikipedia page suggests that the largest real world commercial ships top out at about 5 times the displacement of the largest military vessels, with many commercial vessels three times the size of the largest warships. So if the largest military vessel seen in your Traveller sector is a 60,000 ton cruiser, then the biggest merchant vessel one would expect to find would be 300,000 tons, with 180,000 ton vessels more common.
I've long wanted a rule of thumb like that.
Then inspiration struck me. It was a rust monster.
56 minutes ago