The calculated distance (air line) between Phnom Penh and Sihanoukville is approximately 115 miles respectively 185 kilometers.
By car or train, the actual journey to Sihanoukville is certainly longer, as only the direct route (as the crow flies) between Phnom Penh and Sihanoukville has been calculated here.
Air line (approximately)
185 kilometers
100 nautical miles