I personally would never consider anything under around 125cc but I recently talked to someone who started on a 50cc scooter to do a few mile commute on roads with low speed limits. He said 50cc was plenty for that. Of course, he eventually moved on to bigger faster bikes. Have you read "Sydney to London on a Moped Called Dot"? The author rode quite a ways on a Honda CT110 which on its best days could only do around 80KPH and by the end of the trip struggled to do 50.