How are different items measured?
Barbells are measured from the inside edge of one bead to the inside edge of the other bead. No part of the bead is included in the measurement. So, the length of the barbell is generally the wearable surface of the jewelry.
Beads are measured by their diameter at their widest point.
CBRs are measured by their inner diameter. That is, if a ring is 1/2" but is made of 8ga metal (which has a 1/8" diameter), the inner diameter of the ring will be 1/2", and the outer diameter of the ring will be 3/4". The inner diameter though is the one that is named.