I commuted on a 250 cruiser once, 120 miles a day. 80 mpg was fantastic.
It could do 70 as long as it was on a flat or no stiff headwind.
Lasted a few months until I realized I wanted ABS.
Knowing what I know now, I wouldn't ride below a 650, and prefer a little more. I have learned that torque is king for commuting when you want to get out of tight spots while splitting. If I have to downshift to get to the power I need a bigger engine.
So I prefer big(ish) twins now. 800 is working for me nicely.
But even it won't go from say 70 to 90 very fast (like a 1200 would), but I don't go over 75ish anyway, other folks can have the tickets.
Maybe a nice compromise bike could be say a 500 adv, like a Honda 500X, I rode one of those for a while. It was ok, I upgeared it and it did better on the freeway, but I had to downshift a gear or two to get any power at all.
And I didn't feel comfortable splitting over about 30 mph because it couldn't get you out of a tight spot in anything above 2nd gear.