Sizes can be fairly accurately guessed with a little bit of work.
I did some size estimates on the TEC trade ship last summer since it was tiny by comparison to the other ships and had lots of features like windows and tiered decks to make somewhat accurate size guesses on assuming humans haven't changed much from what we are now to what we are during the age of Sins. Ultimately if you assume the average person is 6 feet tall give or take, then it would make sense that ship deck sizes will remain similar to today's standards at 8 feet tall (2.4 meters). Ignore my labels as they were put there for lore purposes but the drawing itself was done as accurately to scale as I could get it.
Adding in a half foot gap for deck plating as well as utilities (pipes, wires, forced air/heating, etc.) per each deck, floor, level, whatever you want to call it, it averages out to be about 8'6" (2.6 meters). Given the number of decks that are easy to count on the top side of the ship is 5, and remaining decks below it that we cannot easily count appears to be the same height, we can assume that the trade ship has 10 decks. 86' (26.2 meters) from the top of the highest deck to the bottom of the engines. Since that is about 1/3rd the height of the ship, tripling that get's you the total height at 258' (78.6 meters) give or take some footage as it should be expected that I'm not exactly perfect with my scale. The length of the ship is what appears to be two and a half times what it is tall. That would give it a 645' (196.6 meter) length. The width is a little trickier since it appears to be a little shy of half of the height. I'm willing to say it's width is about 40% of it's height, so that would mean it's about 155' (47.2 meters) wide. So there are our dimensions for the TEC trade ship.
Height - 258' (78.6 meters)
Width - 155' (47.2 meters)
Length - 645' (196.6 meters)
If you accept those numbers to be fairly accurate, then all you need to do now is park another ship near a trade ship and start doing some rough size comparisons and you should be able to figure out any other ship or structure from there. That's also if you assume the ships are to scale with each other since I'm fairly certain, maintaining size accuracy was of almost no concern during ship design. ;]