Hi James and welcome. This medium model accommodates (Dog's Neck: 13.5-20.5, Collar Width: 2.0 cm). Cheers and stay safe!

Hi James, this Medium (Dog's Neck: 13.5"-20.5", Collar Width: 2.0 cm)
Other sizes -
X-Small (Dog's Neck: 9"-12.5", Collar Width: 1.5 cm)
Small (Dog's Neck: 11"-16.5", Collar Width: 1.5 cm)
Large (Dog's Neck: 16.5"-24.5", Collar Width: 2.5 cm)
X-Large (Dog's Neck: 24.5"-35.0'', Collar Width: 3.0 cm)
.
Stay safe & have a great day!

Dogs between 11-25 pounds will likely need a collar with a length of 11”-15” (Small) Dogs between 26-55 pounds will likely need a collar with a length of 13”-18” (Medium) Dogs between 56-80 pounds will likely need a collar with a length of 16”-24” (Large)