Will breastfeeding help shrink my uterus to its pre-pregnancy size?

Not a direct effect. Breastfeeding itself probably does not shrink your uterus, but it causes your body to produce Pitocin (oxytocin) which will make your uterus contract. This is natures way of minimizing your postpartum blood loss. By about six weeks postpartum, your uterus will be back to a normal size regardless of whether you breastfeed.