From what I can gather, they're roughly the same width as a person's torso with perhaps a quarter to a half an inch overlap on either side - though, certainly nothing extreme enough to hinder any movement of the arms across the chest.
For obtaining a measurement, I'd suggest measuring a straight line across your chest or hips, and inputting that value as your scale, but you could also measure down from your collarbone to the middle/bottom of your ribcage, and tailor accordingly. You could even use both measurements in tandem to get a more educated guess regarding your scale.
If you need any more help I'd be happy to lend a hand. Good luck with your build!