TWs are mildly tuned so the powerband is surprisingly wide, with a sufficient amount of low speed torque for putting around. Whether or not a TW is sufficient for you depends on how well you know how to ride. If all you know is twist-n-go you'll probably be disappointed. TWs have been ridden to places no other motorcycle has ever been, including the North Pole, the South Pole, and the summit of the tallest peak in Colorado. TWs are one of the most popular bikes in the world for long trips in under-developed areas, including quite a few that have been around the world, though without a fleet of 4x4 support vehicles, helicopters, airplanes, and camera crews for documentation. 12 horsepower is plenty to ride around the world. The rest is just wheelspin.
To answer your question, anything with a wet weight over 300 pounds, a displacement over 350cc, and/or a seat height over 32 inches is to big.