The size in mm refers to the height of the cross. Each cross is sold without necklace.
A crucifix is an image of Jesus on the cross, as distinct from a bare cross. The crucifix is a principal symbol for many groups of Christians, and one of the most common forms of the Crucifixion in the arts. It is especially important in the Latin Rite of the Roman Catholic Church, but is also used in the Orthodox, Oriental Orthodox, Assyrian, and Eastern Catholic Churches, as well as by the Lutheran and Anglican Churches.