as a former owner of a BSA Thunderbolt, 650 was plenty, 750 would have been paradise. At that time, the Vincent was 1000cc and rare, Norton was 750, Honda became 750, and then Kawasaki opened the door to popular large displacement bikes with a 900, and the escalation in displacement was underway. But the mass of riders, harley excepted, did well with 500/650 bikes. Highways were no problem with my thunderbolt. Admission: I've never owned anything larger than my 650, and never felt the need, so take my rambling with a grain of salt.
Cheers!