The calculated distance (air line) between Šentilj v Slov. Goricah and Koper is approximately 121 miles respectively 194 kilometers.
By car or train, the actual journey to Koper is certainly longer, as only the direct route (as the crow flies) between Šentilj v Slov. Goricah and Koper has been calculated here.
Air line (approximately)
194 kilometers
105 nautical miles