The KTM Duke 390 is generally considered suitable for riders of varying heights, including taller riders. With a seat height of approximately 32.7 inches (830 mm), it provides a relatively upright and comfortable riding position. As a rider with a height of 5'11" (around 180 cm), you should be able to comfortably reach the ground and handlebars on the Duke 390.
Factors such as inseam length, torso height, and personal preferences can also influence how well a motorcycle suits an individual rider. It's advisable to visit a KTM dealer or find an opportunity to sit on a Duke 390 to assess its ergonomics and determine if it suits your specific needs and preferences.