If it's longer than 6.5cms, I should be able to help you. My neck is 11cms!
Place the tip of a finger about 4cm to the left of the centre of the chin, then turn the head to the left, keeping the head level, so that point on the jaw-bone is vertically over the collarbone. Now measure vertically down from that point to the collarbone.
Long-necked people give up because of the pain. It doesn't make them any less talented!
What is the maximum height the chin rest can reach?
The standard chin rest goes up to 65 mm above the table of the violin but for an extra £15, a longer screw and ball section can be sent which will raise the chin rest up to 90mms above the table of the violin. Alternatively, the standard screw and ball can be exchanged for a longer one at no extra cost.
It is recommended that the Ergonomic Chin rest is first tried with just a pad underneath. However, some people have found their shoulder-rest works, but only when the Close-coupled version is used.
If you want to play chin-off, your repertoire will be restricted to baroque and early classical. Unless you learn the difficult thumb technique required, shifting down will be impossible.