I'd go with something low maintenance like a Yam XT225, Kaw Super Sherpa, or the new Honda 250.
They're all relatively light, low seat height, moderate power, and decent on the highway.
Learn to ride one of the them, and then decide how much further you want to go in terms of height, weight, and power.
'01 Kawasaki W650
'05 Yamaha XT225