There is an examples of large (12-15"
diameter cryptomeria air layered successfully, so I don't know that there is a practical limit, provided you leave enough tree below the layer to sustain the root system because it won't be being fed by the tree any longer
There are actually two methods for the wire. The faint-of-heart method is to apply a tourniquet around the trunk and just wait for it to grow into the wire
The other method is to ring bark the tree. Cut around the perimeter at two levels, usually a distance apart should be at least 1/2 and up to 1x the diameter of the branch. So about an inch should be ideal in your case. Cut down to wood and remove all the bark between. Then after cleaning up the wound edge with a knife and applying rooting hormone, (the upper edge - lower doesn't matter) optionally a couple of thick diameter wires can be laid right up against the edge of the wound and put on very tightly.
One article has photos of this Japanese guy setting it with a peen hammer all around the trunk. Really pounding it in.
Then there is the damp sphagnum and poly treatment and just be sure it doesn't dry out. It won't need a huge amount of water because the roots aren't there to drink it yet, and the layered branch's needs will be met largely by the translocation of water through the xylem of the wood within.
Layering is a nice way to make a twin trunk maple with trunks right at the base. Ideally they should have a U union and not a V shaped one. That gives the trunks room to grow without growing together.
Hope that hits the spot,
Regards,
Matt