Wax rings are seals placed between the flange and the toilet to prevent leaking. Weight pressure exerted onto the wax ring by the toilet expands the ring so that it quickly forms an effective seal against water leakage. You don’t need to worry about getting the correct size of the wax ring because they are a “one size fits all” component. Some wax rings come with an attached polyethylene or rubber piece that extends into the opening of a toilet flange. Booted wax rings should not be used with toilet flanges that are high enough to prevent the toilet from sitting level on the floor. For flanges that are a little low, thicker wax rings will adequately fill any minimal extra space.