I've had bikes since the early 80s. I've had from 50s to 1100.
I consider a 250 the bare minimum. It's not the bike per se, but rather that you must be in command in traffic. I would say at the beginning the sweet spot is 350-400 .
Later on I believe 500-600 is fine for most people, and even in this range you can find rockets like Rs and RRs.
Now I ride a sport maxiscooter, a Yamaha Tmax, which handles like a dream, has plenty of kick, is happy at 150-160 km/h, and comfortable as hell. This particular bike is the reference in it's segment (which it pioneered) for a reason. My mechanic calls it a proper bike in disguise.
I guess age takes it's toll.