You find a lot of bearing code numbers in KTMs' spare parts catalogues or online at ktminfo.com. The site with the exploded view of your bikes' front wheel says that the whell bearings are one GROOVED BALL BEAR.6003 2RSR.C3 and one GROOVED BALL BEAR.6203 2RSR.C3.
With these bearing numbers you go to the website of a bearing manufacturer, such as
www.skf.de and search in the product tables for the dimensions of 6003 and 6203 barings.
Voilá, your axle is 17 mm in diameter.