17 inches may be correct, but I'd guess could be as long as 19 inches.
Go to Bullberry.com's web site. They build custom Contender and Encore barrels and list data for the .204 Ruger and .17 HMR where starting with 24 inch barrels, loads in each cartridge were chronographed and recorded, then barrels were shortened one inch at a time, readings recorded for each inch.
Optimum length for the .17HMR is 21 inches! 23in for the .204Rug.
Of course the .22WMR is a stright-walled round versus shouldered .17HMR.
I haven't shot the .22WMR, but concur with the 14inches on the .22 Long Rifle round - for optimum most efficient length. Eleven inches is supposed to be optimum for the .22 Short, according to Dr. Marc White from Sound Technologies and his personal experience.