The calculated distance (air line) between Siem Reab and Sihanoukville is approximately 106 miles respectively 170 kilometers.
By car or train, the actual journey to Sihanoukville is certainly longer, as only the direct route (as the crow flies) between Siem Reab and Sihanoukville has been calculated here.
Air line (approximately)
170 kilometers
92 nautical miles