What size baby leads to prolapse of the organs years after vaginal delivery?

No baby: It's just a risk factor not a cause. I performed prolapse surgery on a 70yo nun last year.

No particular size: Number of vaginal deliveries is a risk factor for pelvic organ prolapse. For example, a woman with 6 natural births will be more at risk for prolapse than a woman without any children. The relationship between baby weight and prolapse is still controversial.