If you are looking for actual dimensions to create the symbol, I have never seen any. I have seen slight variations on the symbol (some have the cone more elongated than others). I do not think it really matters as long as the correct projection is represented.
"Everybody is ignorant, only on different subjects." — Will Rogers
I have a copy of BS308 1984. This recommends the proportions of the cone to have an included angle of 30 deg. If the larger dia is d, the cone length is d and the distance between the end of the cone and the vertical centreline of the end view is 1_1/4d.
HTH
Dave