There is an old wives tail out there that the safest boat on Superior is an 18 footer because it fits perfectly in between the swells. More importantly, understanding its weather patterns, your equipment, and safety measures are what should be the determining factor on how big of a boat you need.
My Ranger 619vs is going there next weekend, WEATHER PENDING! Trip plans to there should be tentative until the night before.
Just a few items that can assist in safe travel:
-Marine Radio w/ good coverage
-Use well traveled routes (fairy routes and know their travel times)
-Identify your Gas Stops (there are only 2)
-Plan your route and know your MPG/GPH (Ie. My one way destination is 58.2 miles, @ 40mph and 15gph I will use approx. 21 gal.
-From any single spot (that I go) automatically assume to be 20 miles from gas so at 8 gal remaining its time to turn around.
-Must have up to the minute weather reporting (WX or Satellite)
-Flares, FE, First Aid Kit, Jumpers and dedicated fresh battery, extra floatibles and ropes.
-Travel Partners (extra boats) are nice.
-Know First Aid, CPR, Swimming Skills, etc.
Keep in mind two things:
-Tourney fisherman can run 100 miles to one spot, and there are kayakers that make the 165mi loop around IR. If you are smart and prepared like them, you are good to go.
-If you pull up to your local ramp on a weekend, then unload your gear from your truck and take off your boat cover, then back down, then hook up a rope and wade your boat out to the dock and tie it up, then realize your motor doesnt start, Isle Royale is not the place for you.
Good Luck, Be Safe, and Think Smart with the decision you make.