When I got in to motorcycling, back in the 1960s, they used to say that there was no real reason a motorcycle needed to be bigger than 250cc. This was in the day before bikers were doing long trips on the freeway. In those days motorcycle touring was back roads and secondary roads where you went usually 45 or 50, sometimes as fast as 60. People routinely crossed the US on 250s in those days and wrote travelogs about it.
Modern 250s are MUCH better than the Britbikes we had in those days. More reliable, more long-lived, much better engineered. There is no reason you couldn't take one on a long trip. BUT you can't just get on the freeway and zoom the whole way at 70 mph. You can do 70, but the bike is beating its little heart out. You don't want to go that fast for more than a few miles at a time.
The secondary roads take longer, but they're more fun, and usually more scenic than the super-slab.