I did several hundred miles on my Stella and was fine. Very comfortable, relatively speaking.
Im in my mid 50's now, so I look at road trips differently before. Used to be on bigger bikes and fly down the back roads and highways. Now , I prefer to go slow and stretch the ride out for a while. Stop a few times, take in more of the world and relax. If your route is a good one, and you have time, I cant think of a reason why you wouldn't use a small bike. Sort of related, I tried to trade my Burgman even-up on both a Stella 150 4-t (not the PX150) and 125 in those flat black versions they have come out with, but no dealer will do it. Apparently lots of people think small bikes are the best.