In retrospect, this is one of those "should have seen it coming" things. State building code and local zoning ordinances at the time the property was built apparently do not set any minimum dimensions for a one-car garage and we noticed the garage looked a bit "cozy" when we viewed the place. Also, as the space was being used as a workshop at the time of our offer, the large workbench there made it impossible to try it out for ourselves first.
The city's appraisal database is accurate about the dimensions of the space. I also reviewed the home inspector's report, which states only that the space was "designed as" a one-car garage and makes no representations whatsoever about the space's suitability for a particular purpose (like storing a car!)