Can flatfoot correction surgery make your feet smaller?

Not the goal. The goal of flatfoot reconstructive surgery is not cosmetic; it is meant to correct abnormal function and is a biomechanical condition.
No. Surgery to correct flat feet creates some arch in a foot that has a flattened arch, but should not shrink the size of the foot. Because surgery raises the arch area, that part of the foot should no longer touch the ground as it did when the foot was flat, so the foot may look narrower after surgery.