Your dress form shoulder pad must be metal, or have metal piece under the shoulder pad.
Regarding metal piece questions, please call 1-888-818-1991
Industry Grade Dress Form Left Arm (Magnetic #606B) Size Table
How to measure your arm hole:
Your armhole diameter is the width of your arms when held against the torso, the height of the armhole is only one ingredient for a better range of movement. Check your armhole measurement to get a closer size of PGM dress form magnetic arm.