Having just started myself, I’ll tell you what I’m experiencing.
When I took my MSF course last year, we all rode 250’s. They’re nice and light, easy to manuver and fun for the course. A 250 is definitely a confidense builder if you’re just getting started with riding or it’s been a long time.
I just got a ’82 Yamaha XJ650 which is a bit bigger than most of the starter bikes that are mentioned on this site, however the power is a definite must if you plan to take long trips, live in hilly/mountainous areas, or need to keep up with traffic on the interstate.
A 600 or 650 should be fine if you can handle a bike that’s about 400lbs. I looked at a 900cc bike a month ago and the extra weight is really noticeable, but the 650 I could handle just fine. The factors that will help you choose the right size for you should be what you feel comfortable handling. If you’re bigger a medium sized bike could be just fine, if you’re smaller start smaller. There’s no one who will call you an idiot for knowing your limits, you might get teased by squids or fools, but they are a minority. My friends at work go riding as a group a lot and you’ll see all sizes in their pack and no one gives grief to the falks with smaller bikes.
As far as cruisers go, most of the Yamaha’s I’ve seen have tachometers, but you can tell when to shift pretty easily by sound and intuition.