In regard to your question about 650cc single-cylinders, I bought a Suzuki DR650SE dual sport as my 1st bike. It’s a 650cc single with the bike weighing 324 lbs dry. I had it lowered at the dealership 1.5 inches so I could flat-foot it (I’m 5’9″).
It’s been a great bike to learn on and the power’s very manageable. A lot of fun to fling around. Depending on your comfort with the height of the bike, any of the Suzuki DR series bikes would make good 1st bikes. (DR-Z400S, DR-Z400SM, DR200SE, DR650SE) They all handle great and have manageable power. Plus if you drop them at low speeds there’s typically not much visible damage if any.
Alternatively, The S40 (Formerly Savage) is a good starter bike. Especially for those just starting off who like cruisers and plan to spend a lot of time on the highway. The displacement of the S40 is more suited to maintain highway speeds for long durations than a 250cc cruiser. For you I’d also recommend maybe looking into the Buell Blast. They’re great little urban bikes and have all the cool Buell features like the underslung exhause and belt-driven.
Ultimately, you need to visit lots of dealerships and sit on a lot of bikes. Find something with beginner-manageable power that fits like a glove. A bike won’t do you any good if you get scared when thinking about getting on it for a ride.