So when you say offshore are you thinking 30-50 miles, Gray Zone between Maine and Nov Scotia or are you thinking 100+ miles, Georges and the Canyons?
In the early seventies my dad fished Georges for Western Ocean out of Gloucester. The Western Ocean and Western Wave (still around as a herring boat I think) were steel and in the 75' range. He also ran a wooden converted shrimper the Sea Dog that was 72' I think. Later he fished there for one season on a 35' Ralph Stanley which was the original Miss Julie while the owner was building a 55' Marine Management. Later the current Miss Julie that fishes out of the Sandwich Basin was built as a 68' Aluminum by Gladding & Hearn, current owners stretched it to 72' I think. He made one more season on Georges in late 70's on a 42 Bruno the McCaffery, still inshore lobstering and Pogie fishing out of Hull, MA so it can be done in any size. However, I would say for out there ideally 70'-75" steel seems be the most common and economical. These guys are probably fishing 40 pot trawls and 12-1400 traps, too far to run in after one haul through so may lay to for a day and then rehaul making it a 5-7 +/- day trip. the McKInley fishing out of New Bedford was an Alaskan Crab boat, Similar to the Northwestern with the house forward is 100'+, not sure if it might be too big for the type of fishing but probably the most comfy to work on.
For the 30-50 mile / Gray Zone fishery, minimum of 42' and up to 55' fiberglass. The economics of building and running wouldn't work for a big steel boat I don't think. These guys are limited to 800 traps so if you are trip fishing you are only spending one night offshore and might even work through the night or just do long days if you have enough speed and the economics allow for you to use the speed.