RedactionAnnotation::setFontSize(…) is not working for overlay text.
public void addRedactionAnnotation(Page page, Rectangle rectangle, String overlayText, String colorHex) {
var annotation = new RedactionAnnotation(page, rectangle);
var color = hexToColor(colorHex);
annotation.setFillColor(color);
annotation.setColor(Color.getBlack());
annotation.setModified(new Date());
annotation.setOverlayText(overlayText);
annotation.setTextAlignment(HorizontalAlignment.Left);
annotation.setRepeat(false);
annotation.setFontSize(8);
annotation.redact();
page.getAnnotations().add(annotation);
}