Inner diameter

How are different items measured?

When you are told that a barbell is 1/2" long, what does that really mean? Does it mean the total length, or are they measured from the middle of the bead, or some other way?

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.