I really don't see what is so hard to understand here. New 56' semi-trailers are not legal on the Interstate system ANYWHERE as far as federal rules go. A very few states will allow them on STATE highways only but AFAIK those are grandfathered trailers built before the Feds mandated length. The reason California or any other jurisdiction for that matter won't write a special permit for anything other than a specialized trailer (those haulers are seen as just a dry van)is because every freight company on the planet would be hauling 60' trailers around on permits.

It's pretty simple 53' max length, 41' max kingpin.

Kevin