If you're going to put in two zippers, going from the armpits to the lower back, the easiest way to approximate the length of the zippers you'll need is to put on a well-fitting pair of pants with belt loops. Hike your pants up to your real/natural waist, which is right around your belly button. (It also tends to be the narrowest part of your torso)
Then using a tailor's measuring tape, run it through one of the belt loops on the side of the pants, holding one end into your armpit, and pulling the other end taut until you feel it hit the middle seam on the back of your pants or the central belt loop on the back. (If there is one)
Boom! Measured!
With zippers, it's always better to err on the side of caution. Better to have zippers that are too short (So long as they are longer than the pieces they are attached to) than too long, as that will cause bunching.
Good luck.
-Nick