Q: I could not find in the 2012 Life Safety Code if there is a specific width requirement for a sidewalk as an exit discharge.
A: You are describing the exit discharge, and that is regulated under section 184.108.40.206 of the 2012 LSC, which says yards, courts, open spaces, or other portions of the exit discharge shall be of the required width and size to provide all occupants with a safe access to a public way.
Since the LSC does not specify what the width of the exit discharge should be, it is up to the AHJ to decide. When the building was designed and constructed, those dimensions would have been approved by the AHJ with authority over construction.
But it is mentioned in the NFPA handbook to the 2012 LSC that the exit discharge should be at a minimum wide enough to accommodate the calculated load of people using the exit discharge, so the dimensions used for the interior egress could be expected to be used for the exterior exit discharge. And remember, according to 220.127.116.11, the exit discharge must be level without an abrupt change in elevation greater than ¼ inch, so that means a smooth walking surface is required.