“How much cc will I need to flow with traffic and what would be an overkill for a newbie. One thing for sure, it cannot be too heavy”
The 250cc Kawasaki Ninja will go over 100mph, but it is not the best choice for a passenger or longer trips. The 400cc Suzuki Supermoto is also fast enough for interstates, and it is a little better with a passenger, but that narrow stock seat will feel rough if you do a couple hundred miles plus in one day. The KLR-650 Kawasaki is a better dual sport for long trips and a passenger, but it is quite a bit heavier than the other two bikes listed here, and it is a little too powerful for a beginner, but it is great for someone that started out with dirt bikes and is used to dirt bike style handlebars. People often ride these across South America.
The main things I would consider- how much time are you going to be on the interstates? A 500cc Kawasaki Vulcan cruiser or 500cc Suzuki or Kawasaki sportier bike are probably better choices if you are going to be at 75 mph a lot, and your choices for small bikes is greatly widened if you will mostly stay on 55 mph roads without a passenger. Also, it is wisest to buy an old 250cc dirt bike and learn to slide it sideways on wet grass first- it will save your second bike and your body from some possible major damage while you are learning. One last point to repeat- take the Motorcycle Safety Foundation beginners class first, and see what you think about the bike they give you to train on.
Scooters- very practical, if you can handle the looks of them. Some have the engine mounted on the swingarm, for lower costs and poorer cornering. The 500cc Yamaha T-Max has amazing handling for a scooter, with a sportbike-like frame, and it is quite expensive for a scooter.