Its much better for a beginner to start out on a lightweight low powered bike like the Rebel. These are very forgiving if you make a mistake, and are not fast enough to get a newbie into trouble before they have time to realize what's happening. I always cringe when we get a class with an 18 year old who has a liter sport bike at home waiting for him to get a licence.
Having said that, the Rebel is physically small for a 6 footer. A dual sport 250 or 450 is physically more suitable for a bigger person, and that type of bike is still a great trainer, and great fun to ride.
Lots of people nowadays start out on bigger bikes, even buying their dream Harley as a first bike. I think these folks miss out on a lot by never having the fun of riding a lightweight bike that they can thrash hard without worries. It's true that its much more fun to ride a slow bike fast than to ride a fast bike slow. People that start out on big fast bikes often take a long time to really get a good feel for riding because they are intimidated by the power and weight. If they are not intimidated by the power and weight, some of them come to grief in an accident because of their lack of respect for the bike.
I rolled my 650 triumph up onto the bike lift to do some work on it today, and marvelled at how light and easy it is to roll around. At 375 pounds and 49 horsepower, it would be considered a beginner bike today, but back in 1968 when it was new, it was a fire breathing "big bike" . It was nearly as fast as anything else you could buy back then reputed to be capable of 120 MPH. (hence the model designation T120R) Mine will never get anywhere close to that speed again.
Modern bikes are much better in every way than the old timers, but the old ones do have their nostalgic charms. Its amazing how much power and speed is available today compared to what was possible 50 years ago. To see beginners start out on one of these machines makes me wonder how I would have done had I not gone through the progression of small bikes that I learned on so long ago.
2004 Rebel 250, 2003 BMW K1200GT (roadburner), 2004 BMW R1200GS(all purpose),
1973 Norton Interstate (in a box in the basement)
1968 Triumph Bonneville