With a height of 1.80m (5'11") and long legs, you may find the KTM Duke 125 a bit on the smaller side, but it can still be a suitable option for a first bike. The KTM Duke 125 has a relatively compact design, which may result in a slightly cramped riding position for taller riders.
The seat height of the KTM Duke 125 is around 830mm (32.7 inches), which is manageable for riders of average height. However, due to its narrow profile and the placement of footpegs and handlebars, you may feel a bit more cramped compared to a larger bike.
It's important to consider your comfort and ergonomics when choosing a motorcycle. While the Duke 125 may feel a bit small, it can still accommodate riders of your height. It's recommended to visit a KTM dealer and sit on the bike to assess the comfort level and ensure that you feel adequately in control.
If possible, you can also explore other 125cc options from different manufacturers to see if there are models with more spacious ergonomics that better suit your stature. Remember to prioritize finding a bike that allows you to have a comfortable and confident riding experience.