Have just measured two, for interest, and they were (dia, length, all dims mm) 4.0 x 4.1 and 3.8 x 3.5. Is the 'socket' in the shifter it settles into the limiting factor and what could it cope with max?
Please don't take umbrage if I observe your title is misleading: gear cable has a diameter of 1.2mm and doubt that's changed much.