At 6′-3″ tall, the 250cc and 400cc supermotos, the Kawasaki KLR 650, or the Suzuki M50, C50, or S50 will fit your long legs the best, without any modifications to raise the handlebars, raise the seat, or move the footpegs more forward.
Do you want to do 300 miles plus a day at 75 mph after the first few months? Do you want to carry a passenger later, after you have some experience? Are you mostly going to do city and country riding at 60 mph or less?
The 250 or 500 Ninjas (or 500 Vulcan) are a better choice for doing 75 mph, by yourself for the 250, or the 500 with a passenger, and the Buell Blast is better at 60 mph or less with a passenger. The Kawasaki KLR and Suzuki 50s are more comfortable for long trips, but their higher weights and higher costs are not the best choice for a beginner.
In my opinion, it is best to learn on a dirt bike and then get a street scooter, then a street motorcycle as a third bike, but many people skip these earlier steps in their hurry to have a bigger bike on the street.
Ideally, get the smallest and cheapest bike you can live with, to lower the insurance, gasoline, and maintenance costs, and to be less likely to crash as you learn.
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