Measure per piece
Helmets follow head circumference just above the brows. Chest plates follow chest circumference under the arms and torso length. Gauntlets follow the widest point of the forearm; shins follow the calf.
Add wearing clearance
Rigid prints need more room than foam: roughly 60 mm around the chest and 30 mm around the head, plus about 2π × your padding thickness. The AI Fit Advisor works this out per armor type.
Turn numbers into a scale
Load your file in the Sizing Studio, pick the part and it compares your measurement plus clearance to the model's real size, per axis.
Next: more armor guides.