No, there is not a standard width for Military Training Routes (MTRs).
MTRs are developed for use by the military for low-altitude, high-speed training. The routes above 1,500 feet above ground level (AGL) are developed to be flown under IFR. The routes at 1,500 feet AGL and below are generally developed to be flown under VFR.
Operations on “IR” routes are conducted in accordance with IFR rules regardless of weather conditions while operations on “VR” routes are conducted under VFR rules except flight visibility must be 5 miles or more and flights must not be conducted below a ceiling of less than 3,000 feet AGL.
MTRs will be identified and charted as follows:
MTRs with no segment above 1,500 feet AGL must be identified by four number characters; e.g., IR1206, VR1207.
MTRs that include one or more segments above 1,500 feet AGL must be identified by three number characters; e.g., IR206, VR207.
There is not a standard width for MTRs which is why you don’t see the information published in the AIM. According to military documents, they can be as narrow as 3nm either side of centerline or as wide as 8nm wide from centerline and widths can vary along the airway itself.