I can't tell you where to find the info, but I can tell you that there is no mention of the dimensions of helicopter routes in the Aeronautical Chart User's Guide (8th edition) which is available for download on the naco.faa.gov web site.
However, on the MTAs it says the following:
MTRs can vary in width from 4 to 16 miles.
Detailed route width information is available in the Flight
Information Publication (FLIP) AP/1B (a DoD publication),
or in the
Digital Aeronautical Chart Supplement (DACS)
produced by NACO.
So maybe the DACS will also shed some light on the helicopter route issue. Could be completely wrong though. My guess is that you have to be over the depicted route to be on the route
