Best first sport bike for tall rider?

Check out the Kawasaki 650R. Its a v-twin engine, so it wont be as powerful or twitchy as a supersport's inline 4, but it definitely has power you can grow into, unlike a 250R. It also has a more upright seating position as opposed to leaned over, so you might find it more comfortable than other sport bikes.

Having dirt bike experience puts you in a much better position for riding a crotch rocket, you will do fine on a 600 as long as you respect the bikes power until you get used to it and dont do what you wouldnt do on an equivalent dirt bike. I would STRONGLY recommend you not waste your money on a 250 as you will tire of it quickly because it will be way too slow and boring in less than 1 month.
Suzuki has a gs500 that is tuned towards beginner riders