What is the width of a high altitude airway? Victor airways (airways that are below 18,000 ft) has a width of 8 NM. How about those airways that operates higher than 18,000 ft, do they have specific width you must be in?  

Or is there no width because you would be using GPS? (ones with an RNP of 2 NM)
At least in the US airspace definition, High Altitude Airways (Jet Routes) have no defined width regardless of the airway being navaid-based or RNAV/RNP

This is briefly stated in the FAA training manuals:
http://tfmlearning.faa.gov/Publications/atpubs/AIR/air2004.html