I have not actually made my bicep armor yet, but my measuring method would probably work best. Take an actual tape measure (the one most commonly used to tailor make suits, not the self-retracting ones), and wrap it around the thickest part of your bicep. While holding the tape, move your arm in the full range of motion that it can, letting the tape expand if it needs to, this will give you the minimum your inside diameter must be in order to allow full-range motion. Do some math and viola! measurements abound. Hope this helps.